Since we are getting the new optional class feature Dedicated Weapon in TCoE, I want to ask if it would work with a Sun Blade.

The feature allows to count a weapon as monk weapon, if it has the following requirements:

- must be a simple or martial weapon

- you must be proficient with it

- it must lack the heavy and special properties

The Sun Blade does count as proficient if you are proficient with Shortswords or Longsword so technically it would work right?

I think you'd still need longsword proficiency.

My reasoning is that what you're being asked to choose is a simple or martial weapon type, which then counts as a monk weapon for you. Shortsword proficiency being usable with the Sun Blade doesn't actually make it a shortsword, it's still a longsword, so choosing shortswords as a monk weapon wouldn't include the Sun Blade. Meanwhile Sun Blade isn't a weapon type in it's own right (I'm pretty sure only weapons listed in the player's handbook equipment section are valid weapon types for this sort of thing).

You can obtain longsword proficiency through your choice of race (and in Tasha's Cauldron can swap racial weapon proficiencies if you want I believe), or you can take a feat or multi-class. You can also use longswords (and longbows) as part of the Way of the Kensei.

In practice I doubt many DM's would rule it this strictly, but I think RAW it's a longsword, it just happens to be light enough that shortsword proficiency is sufficient to wield it effectively.

UPDATE: Nevermind me, I didn't realise this feature had changed from UA (where you picked a number of weapon types as dedicated weapons), since you now just touch one weapon then yes, a Sun Blade absolutely counts.

If the Sun Blade grants you proficiency as a shortsword or longsword, and shortswords are already count as monk weapons, you might not need to use the Dedicated Weapon feature. This depends a bit on your DM's interpretation how to interpret a Sun Blade as weapon by its own or a special form of a shortsword or longsword. It is not so clear in its description as the Luck Blade, which defines the different forms of Luck Blades by a list given in its description and by this the appropriate proficiency.

The DDB char sheet supports the Sun Blade as a monk weapon when you select it as a dedicated weapon, whereas prior to TCoE you could not do this.

Quote from Faerik >>

If the Sun Blade grants you proficiency as a shortsword or longsword, and shortswords are already count as monk weapons, you might not need to use the Dedicated Weapon feature. This depends a bit on your DM's interpretation how to interpret a Sun Blade as weapon by its own or a special form of a shortsword or longsword. It is not so clear in its description as the Luck Blade, which defines the different forms of Luck Blades by a list given in its description and by this the appropriate proficiency.

 The Sun Blade is listed as a longsword but you can have short or longsword proficiency to wield it, so a Monk could already use it just not as a Monk weapon since technically it's a longsword. By making it a Dedicated Weapon you pretty much just solve the problem of the 3rd point of the bonus action attack. The first two are moot because of it being a finesse weapon & its damage is already higher than your Martial Arts damage. Dedicated weapon gives you more bang for your buck from a mundane or magical longsword that require STR & are not finesse. At 2nd level an Elf could make a mundane longsword their Dedicated Weapon & start doing more damage than what they were doing with that spear or quarterstaff they used at level 1!

• You can use Dexterity instead of Strength for the attack and damage rolls of your unarmed strikes and monk weapons.
• You can roll a d4 in place of the normal damage of your unarmed strike or monk weapon. This die changes as you gain monk levels, as shown in the Martial Arts column of the Monk table.
• When you use the Attack action with an unarmed strike or a monk weapon on your turn, you can make one unarmed strike as a bonus action. For example, if you take the Attack action and attack with a quarterstaff, you can also make an unarmed strike as a bonus action, assuming you haven’t already taken a bonus action this turn.

You wouldn't need to use the dedicated weapon feature as the sunblade is considered both a short sword and a long sword. So a base monk has the ability to wield it with no problems

Quote from SIthwarlock >>

You wouldn't need to use the dedicated weapon feature as the sunblade is considered both a short sword and a long sword. So a base monk has the ability to wield it with no problems

 Some would disagree. It only allows you to be proficient if you have proficiency in shortswords. It isn’t actually a short sword so not a monk weapon 

“While the blade exists, this magic longsword has the finesse property. If you are proficient with shortswords or longswords, you are proficient with the sun blade.”