So random situation I thought of and wanted to know what the ruling would be. 

Situation: A player with adamantine armor and an AC of 25 is fighting a creature with an attack bonus of +4. Adamantine Armor makes it so that a critical hit is treated as a normal hit.

RAW from the player handbook, "If the d20 roll for an attack is a 20, the  attack hits regardless of any modifiers or the target's AC. In addition, the attack is a critical hit..."

Question: Does Adamantine Armor negate the ability of a natural 20 to hit the wearer IF the attack roll is below the wearer's AC. (In the above situation, the creature with the +4 attack bonus would only have a 24 to hit, less the player's 25 AC)

No. A normal hit is still a hit.

Not according to the RAW quote you just posted. For there to be an argument that Adamantine would negate a nat. 20 bring an automatic hit, the rules would have to read something like "If the d20 roll for an attack roll is  a 20, the attack is a critical hit....critical hits hit their target regardless of the targets AC". This makes the automatic hit a property of being a critical hit, which the adamantine armor would negate. This is not what the rules say though. As it stands, the property that makes the attack an automatic success is that the die face says 20, NOT that the attack is a critical hit. Therefore, since adamantine armor does not change the die face, the attack should still be an automatic hit.

Quote from dwaller2014 >>

Not according to the RAW quote you just posted. For there to be an argument that Adamantine would negate a nat. 20 bring an automatic hit, the rules would have to read something like "If the d20 roll for an attack roll is  a 20, the attack is a critical hit....critical hits hit their target regardless of the targets AC". This makes the automatic hit a property of being a critical hit, which the adamantine armor would negate. This is not what the rules say though. As it stands, the property that makes the attack an automatic success is that the die face says 20, NOT that the attack is a critical hit. Therefore, since adamantine armor does not change the die face, the attack should still be an automatic hit.

Yeah, I don't know if OP just misquoted or if it's received errata, but the current text on D&D Beyond is clear that the auto-hit is part of the critical hit, not a separate thing that also happens on a natural 20. The point is that it doesn't matter: adamantine armor explicitly turns a critical hit into something that is still a hit.

The following protects against a Natural 20 always hitting.

1. Disadvantage (unless you get a double nat 20)
2. Redirect the attack to another target.  Examples:   Mirror Image spell, Mounted Combat feat
3. Being protected by something like a Wall of Force that prevents attacks.

Adamantine does not.  It just stops crits

I agree with all comments, valid points on each, and would want to add this citation as well: https://www.dndbeyond.com/sources/basic-rules/introduction#TheD20 ...

"In cases where the outcome of an action is uncertain, the Dungeons & Dragons game relies on rolls of a 20-sided die, a d20, to determine success or failure."

If the roll of a 20 on a d20 results in a failure, why bother rolling it? The general rule that a 20 always hits is the answer. This is specifically for an attack roll.

I'm fully aware that there is no such thing as a "critical" for ability checks or saving throws, but in that same thought, if a 20 doesn't result in a success, why was the roll even asked for? The outcome of that action would not be uncertain. It is certain, you fail. Don't lift the dice as a player or entertain rolls as the DM if the outcome is certain.

Raw says a 20 on the die is a critical hit if you are immune to those that would make it not a critical and the modifiers as normal against your ac

I'd like to share some conversations with the Dev about Adamantine Armor and Critical Hits, in case they're useful for anyone:

@usmcbears Does the nat 20 still auto hit against adamantine armor? 
@JeremyECrawford Yes.

@MGreat321 New Print read like this: N20=ignore modifiers and AC= Critical hit(also 2xdamage). Where as original read like N20=ignore modifiers and AC+Critical hit。
The new print seem to imply crit=ignore modifiers and AC and roll 2xdamage. Where as old crit=2xdamage.
@JeremyECrawford As the person who wrote both versions of that rule, I can tell you that the rule means what it says: rolling a 20 is what ignores AC. The 20 is also called a critical hit, the effect of which is explained in the critical hit rule.

@TheyCallMeTomu Adamantine Armor negates critical hits. The crux of the question is, does a natural 20 automatically hit as a result of being a natural 20, or does it automatically hit as a result of being a critical hit. Previous editions were clearer on this point.
@JeremyECrawford "If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC" (PH, 194).

@MGreat321 I know N20 is a critical hit and how the critical hit works when you roll damage. But ignoring modifiers and target AC are not tie to the critical hit in the original PHB, now srd and dndbeyond seem to imply that they do.
@JeremyECrawford In every version of the PH, rolling a 20 is what causes you to automatically hit.

@MtS_Designer So to conclude: A champion fighter roll a 19 against a creature with 20 AC wearing an adamantine armor. The fighter will still auto-hit that target even if this phenomenon of auto-hitting is called a critical hit (which the armor gives immunity).
@JeremyECrawford Yes, that fact hasn't changed since the core books were published.

Quote from TarodNet >>

 

@MtS_Designer So to conclude: A champion fighter roll a 19 against a creature with 20 AC wearing an adamantine armor. The fighter will still auto-hit that target even if this phenomenon of auto-hitting is called a critical hit (which the armor gives immunity).
@JeremyECrawford Yes, that fact hasn't changed since the core books were published.

Champion, or other features and abilities that give you a crit on rolls other than a 20, are where you really have to thread the needle on the rules in 5e24

Critical HIt

If you roll a 20 on the d20 for an attack roll, you score a Critical Hit, and the attack hits regardless of any modifiers or the target’s AC.  A Critical Hit lets you roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers.

Champion - Improved Critical

Your attack rolls with weapons and Unarmed Strikes can score a Critical Hit on a roll of 19 or 20 on the d20.

Now, does rolling a 20 mean you get a Critical Hit, which always hits? Or does rolling a 20 mean your attack always hits, and it's a critical (two separate effects)?

If it's the former, then a Champion still auto-hits on a 19, because a crit is an auto-hit. If it's the latter, then they don't, because their feature doesn't say "treat a 19 like a 20", it says they get a crit on a 19 -- which is only one of the two effects of rolling a 20

You score a Critical Hit, and the attack hits regardless of any modifier to me means 20 or other roll if said otherwise.

Quote from EonDestroyerDragon >>

Raw says a 20 on the die is a critical hit if you are immune to those that would make it not a critical and the modifiers as normal against your ac

 For the attack to be a critical hit against you, it must hit you. Therefore for the armor to modify the critical hit to a normal hit, you have already been hit by it

The item only modifies the effect on damage from a critical hit, not the effect on hitting (ie insta-hit). Otherwise you'd have a scenario where an item that modifies a hit against you (not an attack, a hit) into not being a hit against you therefore it no longer modifies it, therefore it hits you.

It's always been the case that anything that modifies a critical hit into a normal hit (or vice versa) applies only to the damage.

If something would modify the attack, it would say it modifies the roll.

Quote from Plaguescarred >>

You score a Critical Hit, and the attack hits regardless of any modifier to me means 20 or other roll if said otherwise.

Except the Glossary entry has a comma between the 'Critical Hit' and 'auto-hit' phrasing, which at least implies that they are different things, and it also includes a definition of a Critical Hit which doesn't mention auto-hits

It would have been very easy to write the section like this instead if they meant for a crit to be an auto-hit:

If you roll a 20 on the d20 for an attack roll, you score a Critical Hit.  A Critical Hit always hits regardless of any modifiers or the target’s AC, and it lets you roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers.

Auto hit is not a distinct rules but an effect when scoring a Critical Hit. Just like doubling damage dice, which also occur on a 19 let's concur.

Quote from AntonSirius >>

 [...] Now, does rolling a 20 mean you get a Critical Hit, which always hits? Or does rolling a 20 mean your attack always hits, and it's a critical (two separate effects)?

If it's the former, then a Champion still auto-hits on a 19, because a crit is an auto-hit. If it's the latter, then they don't, because their feature doesn't say "treat a 19 like a 20", it says they get a crit on a 19 -- which is only one of the two effects of rolling a 20

 IMO, it's the former.

A similar question was asked in this thread: Are extended range crits automatic hits? 

Slightly related because it lets us view the issue through the lens of D&D's history, at least 3e/3.5e didn't score crits unless you hit first, and only a nat 20 guaranteed a hit. (However, rules can change between editions, don't take this as a definitive answer.)

This is a strange case where the main text and the Rules Glossary entry do not actually say the same thing.  As such, one or the other needs errata.

Main Text:

If you roll a 20 on the d20 (called a “natural 20”) for an attack roll, the attack hits regardless of any modifiers or the target’s AC. This is called a Critical Hit.

. . .

When you score a Critical Hit, you deal extra damage.

Rules Glossary:

If you roll a 20 on the d20 for an attack roll, you score a Critical Hit, and the attack hits regardless of any modifiers or the target’s AC.

. . .

A Critical Hit lets you roll extra dice for the attack’s damage against the target.

According to the main text, any attack that always hits "is called" a Critical Hit.  By extension, if you made an attack that scored a Critical Hit, that means that the attack must have been one that always hits.  By default, this happens when you roll a natural 20.

According to the Rules Glossary, it is the rolling of a natural 20 that causes you to score a Critical hit.  The rolling of a natural 20 also causes your attack to always hit.

This is problematic in the case of the Champion Fighter:

Your attack rolls with weapons and Unarmed Strikes can score a Critical Hit on a roll of 19 or 20 on the d20.

So now, this character rolls a natural 19.  According to this feature, this attack roll "can score" a Critical Hit. 

If we follow the rules of the main text, a Critical Hit is defined as an attack that always hits.  Therefore, this attack always hits.

But if we follow the rules of the Rules Glossary, nothing special happens on a natural 19.  If there is damage, we can roll extra dice for the damage, but there won't be any damage unless the attack hits and there is nothing about this rules interaction that guarantees that this attack roll resulted in a hit.

In my opinion, the wording of the main text is the RAI.

Quote from up2ng >>

This is a strange case where the main text and the Rules Glossary entry do not actually say the same thing.  As such, one or the other needs errata.

Main Text:

If you roll a 20 on the d20 (called a “natural 20”) for an attack roll, the attack hits regardless of any modifiers or the target’s AC. This is called a Critical Hit.

. . .

When you score a Critical Hit, you deal extra damage.

 

Rules Glossary:

If you roll a 20 on the d20 for an attack roll, you score a Critical Hit, and the attack hits regardless of any modifiers or the target’s AC.

. . .

A Critical Hit lets you roll extra dice for the attack’s damage against the target.

 

According to the main text, any attack that always hits "is called" a Critical Hit.  By extension, if you made an attack that scored a Critical Hit, that means that the attack must have been one that always hits.  By default, this happens when you roll a natural 20.

According to the Rules Glossary, it is the rolling of a natural 20 that causes you to score a Critical hit.  The rolling of a natural 20 also causes your attack to always hit.

This is problematic in the case of the Champion Fighter:

Your attack rolls with weapons and Unarmed Strikes can score a Critical Hit on a roll of 19 or 20 on the d20.

 

So now, this character rolls a natural 19.  According to this feature, this attack roll "can score" a Critical Hit. 

If we follow the rules of the main text, a Critical Hit is defined as an attack that always hits.  Therefore, this attack always hits.

But if we follow the rules of the Rules Glossary, nothing special happens on a natural 19.  If there is damage, we can roll extra dice for the damage, but there won't be any damage unless the attack hits and there is nothing about this rules interaction that guarantees that this attack roll resulted in a hit.

In my opinion, the wording of the main text is the RAI.

 Specific rules > General rules. 

Quote from MPA >>

 Specific rules > General rules. 

What's your point?  Yes, that is a true statement.  It's also totally irrelevant to this discussion.

There are not many cases in my campaigns where the attacker has the effect of +1 to crit (Champion) fighting something in adamantine armor. No matter how the crit occurs, against admantine armor, it is no longer a crit.

I don't mind siding with player and allowing the 19 to be the same as a crit in every respect - auto hit regardless of AC and bonus damage.

Sure there is a chance they will miss naturally in spite of having a probable +5 or better to-hit meaning they would score a hit on AC 24+ anyway. 

**does an unconscious PC in adamantine armor cancel the autocrit of melee attack on an unconscious target?