So, if I were to use the disarming strike and make an opponent drop his special +1 sword, then take a free action to pick up his sword, can I use that sword for the rest of my attacks that round?

Yes, I think that works.

This gives off heavy "my DM wouldn't let me do this thing so I'm going to see what the internet has to say about it" vibes. Your DM's rule is final at your table regardless of what a bunch of people on the internet say.

That being said, drawing or sheathing a weapon is a part of your attack action. Using an object, which picking up a weapon (especially an enemies weapon) off of the ground is, would be its own action. So no, you could not do what you described

You don't think picking up a weapon from the floor would count as part of this?

You can also interact with one object or feature of the environment for free, during either your move or your action. For example, you could open a door during your move as you stride toward a foe, or you could draw your weapon as part of the same action you use to attack.

You would have already had to use that in order to draw your weapon or to stow/get rid of your weapon in order to use someone else's in you weapons place. There is also the matter of it being someone else's weapon and thusly being inside of their space and potential weirdness that comes with that. If you had no weapon in your hand and did this I would say it is okay or if you disarmed you opponent then started your next turn by taking their weapon, but to do it in a single turn you would be abandoning one weapon(the one you attacked with) and ALSO getting out a different weapon that wasn't in your possession. It seems like too much to do within the wording of the action and the time frame (6 seconds). Again, the problem isn't with using someone elses weapon to attack, its with disarming them, getting rid of your weapon, then continuing your attack with their weapon all as a part of the attack action. 

Quote from ConalTheGreat >>

You would have already had to use that in order to draw your weapon or to stow/get rid of your weapon in order to use someone else's in you weapons place. There is also the matter of it being someone else's weapon and thusly being inside of their space and potential weirdness that comes with that. If you had no weapon in your hand and did this I would say it is okay or if you disarmed you opponent then started your next turn by taking their weapon, but to do it in a single turn you would be abandoning one weapon(the one you attacked with) and ALSO getting out a different weapon that wasn't in your possession. It seems like too much to do within the wording of the action and the time frame (6 seconds). Again, the problem isn't with using someone elses weapon to attack, its with disarming them, getting rid of your weapon, then continuing your attack with their weapon all as a part of the attack action.

Ok sure, there are mitigating circumstances when you might not be able to do it, but assuming you still have a free item interaction left and a free hand with which to do it, this is completely legal. Disarming strike can be done with an unarmed strike (it says a weapon attack and as we are told in the Sage Advice Compendium, unarmed strikes are melee weapon attacks) so that lets a Battle Master do the really cool move of making a disarming attack with their bare hands, catching the weapon in midair as their item interaction, and then following through with the rest of their Extra Attack using the weapon they just took away from their opponent. Cool martial arts!

If that was the situation, then totally. However, that seems like a really niche thing and much less common an acurance then attacking someone with a weapon to disarm them. Unless you are a monk or using a Tasha's fighting style, you would be sacrificing damage and the chance to hit in order to attempt to take the persons weapon. I say more often then not you are not going to be in a situation where it is possible.

But again, DM has the final say. 

Quote from ConalTheGreat >>

This gives off heavy "my DM wouldn't let me do this thing so I'm going to see what the internet has to say about it" vibes. 

 To be fair, the topic of disarming itself is kind of a contentious thing. Aside from this specific Battlemaster power, it's barely addressed at all in the rules despite being extremely effective against certain enemies and many PCs. I wish they'd either given disarming robust rules and developed content with it in mind or just not allowed it at all.

Quote from Ophidimancer >>

Quote from ConalTheGreat >>

You would have already had to use that in order to draw your weapon or to stow/get rid of your weapon in order to use someone else's in you weapons place. There is also the matter of it being someone else's weapon and thusly being inside of their space and potential weirdness that comes with that. If you had no weapon in your hand and did this I would say it is okay or if you disarmed you opponent then started your next turn by taking their weapon, but to do it in a single turn you would be abandoning one weapon(the one you attacked with) and ALSO getting out a different weapon that wasn't in your possession. It seems like too much to do within the wording of the action and the time frame (6 seconds). Again, the problem isn't with using someone elses weapon to attack, its with disarming them, getting rid of your weapon, then continuing your attack with their weapon all as a part of the attack action.

Ok sure, there are mitigating circumstances when you might not be able to do it, but assuming you still have a free item interaction left and a free hand with which to do it, this is completely legal. Disarming strike can be done with an unarmed strike (it says a weapon attack and as we are told in the Sage Advice Compendium, unarmed strikes are melee weapon attacks) so that lets a Battle Master do the really cool move of making a disarming attack with their bare hands, catching the weapon in midair as their item interaction, and then following through with the rest of their Extra Attack using the weapon they just took away from their opponent. Cool martial arts!

 "Free item interactions" only occur as part of a move or action. It isn't something separate from either of those two things.

Quote from Farling >>

 "Free item interactions" only occur as part of a move or action. It isn't something separate from either of those two things.

Um, okay?

Quote from scatterbraind >>

Quote from ConalTheGreat >>

This gives off heavy "my DM wouldn't let me do this thing so I'm going to see what the internet has to say about it" vibes. 

 To be fair, the topic of disarming itself is kind of a contentious thing. Aside from this specific Battlemaster power, it's barely addressed at all in the rules despite being extremely effective against certain enemies and many PCs. I wish they'd either given disarming robust rules and developed content with it in mind or just not allowed it at all.

 They do have it in one of the rule books, i believe its in the DMG as additional optional actions the players can take. Linked here. To your point, I have never seen these actions discussed and considering that they are in the DMG its very reasonable for players to never know about them. I mean, I didn't until i started looking into disarming for this conversation, and I have been the DM for my table for over 3 years.

Anyway the ruling on the Disarm Action: "A creature can use a weapon attack to knock a weapon or another item from a target’s grasp. The attacker makes an attack roll contested by the target’s Strength (Athletics) check or Dexterity (Acrobatics) check. If the attacker wins the contest, the attack causes no damage or other ill effect, but the defender drops the item. The attacker has disadvantage on its attack roll if the target is holding the item with two or more hands. The target has advantage on its ability check if it is larger than the attacking creature, or disadvantage if it is smaller."

Quote from ConalTheGreat >>

You would have already had to use that in order to draw your weapon or to stow/get rid of your weapon in order to use someone else's in you weapons place. There is also the matter of it being someone else's weapon and thusly being inside of their space and potential weirdness that comes with that. If you had no weapon in your hand and did this I would say it is okay or if you disarmed you opponent then started your next turn by taking their weapon, but to do it in a single turn you would be abandoning one weapon(the one you attacked with) and ALSO getting out a different weapon that wasn't in your possession. It seems like too much to do within the wording of the action and the time frame (6 seconds). Again, the problem isn't with using someone elses weapon to attack, its with disarming them, getting rid of your weapon, then continuing your attack with their weapon all as a part of the attack action. 

 Dropping an object is not considered to use your free item interaction. So, if you had a rapier in your hand and disarmed an opponent of their +1 Rapier, then it is absolutely RAW that you could drop your Rapier for free and pick up their +1 Rapier as your free interaction to use for your second attack. You just have to do it the turn after you’ve drawn your Rapier. Let them pick up my shabby old normal rapier if they want to, I’ve gotten myself a better one.

This feels like the sort of thing that's more table-dependent than strict RAW/RAI reading.  For some groups, the whole "I kick his sword out of his hand, snatch it out of the air, and then use my other attack to carve my initials in his chest!" thing is just part of the high-flying swashbuckling bravado adventure they're going for.  For other tables, that sort of cinematic nonsense would just get in the way of the more serious tone they're trying to hold to for their games.

With my DM Hat on?  I'd be inclined to let it happen if the player was playing the sort of character for whom Flynn-esque cinematic swordplay was a thing, and if that player was trying to make a Memorable Moment rather than cheese my game.  Probably involve a Sleight of Hand check somewhere to see if they can successfully gain control of the sword well enough to turn around and attack with it in the same turn, or if it takes up the rest of their attack space to get a fumble under control.  It's a potential Really Cool Moment, balanced with the drawback of stealing an Evil Baddite's magic sword being a fan-gorram-tastic way of getting curses.

The issue for me wouldn't be the use of your free action but where the dropped sword is located. You can use a free action to pick up something in the ground, draw a sword, etc. as long as it's on your person or very close by like in your 5' square. If that is something one of my players would like to try I would probably roll to see where the sword landed and then have them do a skill check to see if they could quickly grab the weapon if it is within reach. But just because it was dropped does not mean you can just pick it up.

 Well, the flip side of all this "Nah bro, that's the other guy's sword, you can't have it" stuff - which is valid - is that in 5e?  Due to weird confluences of rules, being disarmed means almost nothing.  A creature that is disarmed can use its free interaction to re-arm itself and then proceed to attack as if it never dropped its weapon, unles an entirely different creature than the disarmer uses an ENTIRE ACTION - the bulk of their whole freaking turn - to kick the critter's weapon away.  And even then, some DMs will push back and try to prevent the kicking-away in the first place because there's no provision RAW for kicking something away from something else.

It turns what should be a Serious Freaking Problem, a.k.a. being disarmed and forcibly losing control of your weapon - into a complete nothingburger, and that just doesn't feel good to the Battlemaster who burned a maneuver choice and one of their Superiority die on that awesome disarming attack.  The Battlemaster gets rid of the enemy's dangerous magical weapon to make the fight easier - except whoops, oh no, the enemy just picked it back up for total freebies like the disarm never happened.

At the very least, I encourage DMs to allow someone who disarms a target to spend one of their remaining attacks, or perhaps their bonus action, to kick the object away if they so desire, instead of insisting it be the entire action of a second player.  Someone who invests in enhanced disarming capabilities should be able to actually disarm their target in a way that has impact.  And hey - if disarming targets is actually possible, baddites can do it to the players, too!  Remind the PCs why everybody is supposed to have at least one spare dagger on them at all times, make them appreciate their fancy Sword of Winning all the more for having to do without it once in a while.

Lucky for me I don't have to deal these kinds of shenanigans at my table. If there is a weapon on the ground or in a rack or otherwise not in your possession, it takes your action to grab it.

The reason for this being, if you went through the trouble of disarming somebody and didn't have a followup attack, then the guy you just disarmed could pick up his weapon with no penalty and attack you with it. 

I'm not saying that it is impossible, I'm saying that having to pick up something off of the ground is not a part of arming themselves. It is an interact with an object action. Feasibly if you disarm a target they are going to do their best to stop you from taking away their weapon. allowing you to pick up something off the ground as a part of the free action to arm yourself, would be the same as saying that you can make three attacks with your sword while also pulling the lever on the wall to open the door, or hitting with your two handed axe, throwing it away, pulling out a two handed crossbow, throwing that away, picking up a chair and smashing it over someone's head all as a part of a single action. It seems a bit much to me to be swapping like that. 

Quote from Yurei1453 >>

It turns what should be a Serious Freaking Problem, a.k.a. being disarmed and forcibly losing control of your weapon - into a complete nothingburger, and that just doesn't feel good to the Battlemaster who burned a maneuver choice and one of their Superiority die on that awesome disarming attack.  The Battlemaster gets rid of the enemy's dangerous magical weapon to make the fight easier - except whoops, oh no, the enemy just picked it back up for total freebies like the disarm never happened.

At the very least, I encourage DMs to allow someone who disarms a target to spend one of their remaining attacks, or perhaps their bonus action, to kick the object away if they so desire, instead of insisting it be the entire action of a second player.  Someone who invests in enhanced disarming capabilities should be able to actually disarm their target in a way that has impact.  And hey - if disarming targets is actually possible, baddites can do it to the players, too!  Remind the PCs why everybody is supposed to have at least one spare dagger on them at all times, make them appreciate their fancy Sword of Winning all the more for having to do without it once in a while.

I would disagree on that nothingburger point. If you decide to move away then an op attack would be without the weapon for one. Also, taking someone's main weapon is a HUGE thing so doing a disarming strike then having someone swoop in a grab that sucker and run away with only a fist as an op attack can be a major power swing in a battle. That should take more than just a superiority die and need coordination with other characters. Having disarming strike is a big deal but should not be combined with being able to snatch up the weapon and use it against them in the same turn, that would be a bit OP. 

I am making a character, not a previous ruling from my DM.  I am taking the unarmed fighting style, but will have a club as well.  My wondering is, when I hit level 3 and pick a subclass, and if I picked battlemaster... can I use the disarming strike to do something like this.

Because as it stands, if I disarm an opponent, that npc is able to use their free action to pick up their weapon and attack the next turn.  The best way I can think of is to either pick up the weapon (as my free interaction) and attack with it, or just hold it.  Or I could possibly take the grappler feat (or tavern brawler) and then as a bonus action grapple them and drag them away from the item at half speed.  

What I have in my mind is a special move in which an unarmed fighter attacks and steals their opponents own weapon to kill them with it.  Real ninja like.

Quote from KrisLewis >>

I am making a character, not a previous ruling from my DM.  I am taking the unarmed fighting style, but will have a club as well.  My wondering is, when I hit level 3 and pick a subclass, and if I picked battlemaster... can I use the disarming strike to do something like this.

 

Because as it stands, if I disarm an opponent, that npc is able to use their free action to pick up their weapon and attack the next turn.  The best way I can think of is to either pick up the weapon (as my free interaction) and attack with it, or just hold it.  Or I could possibly take the grappler feat (or tavern brawler) and then as a bonus action grapple them and drag them away from the item at half speed.  

What I have in my mind is a special move in which an unarmed fighter attacks and steals their opponents own weapon to kill them with it.  Real ninja like.

It's completely rules legal, but as you can see, some people don't like it and would rule against it.