The brace maneuver from Tasha's reads:
When a creature you can see moves into the reach you have with the melee weapon you're wielding, you can use your reaction to expend one superiority die and make one attack against the creature, using that weapon. If the attack hits, add the superiority die to the weapon's damage roll.
Can I use this with an unarmed strike, or do I have to be wielding an actual weapon?
The Brace Maneuver states:
When a creature you can see moves into the reach you have with the melee weapon you're wielding, you can use your reaction to expend one superiority die and make one attack against the creature, using that weapon [...]
You cannot wield an unarmed strike. I come to this conclusion because the rules are written using natural language and if somebody told me they were wielding their head, foot, or hand, I would be very confused. However, there is a stronger argument to make:
Regardless of whether or not you can wield an unarmed strike, the feature requires a weapon at two different times and unarmed strikes are not weapons:
[...] Instead of using a weapon to make a melee weapon attack, you can use an unarmed strike: a punch, kick, head-butt, or similar forceful blow (none of which count as weapons). [...]
Because unarmed strikes are not themselves weapons, they cannot be used for the Brace Maneuver.
Unarmed strikes are not weapons
The Combat section of the Player's Handbook explains unarmed strikes:
Instead of using a weapon to make a melee weapon Attack, you can use an Unarmed Strike: a punch, kick, head--butt, or similar forceful blow (none of which count as weapons). On a hit, an Unarmed Strike deals bludgeoning damage equal to 1 + your Strength modifier. You are proficient with your unarmed strikes.
So you can see that unarmed strikes are NOT considered weapons and therefore, strictly Rules As Written, are not a valid option for the Brace maneuver.
