Can the damage from the Hunter's Mark spell be added to the damage from the Unarmed Fighting fighting style at the start of the character's turn?

Question

The description of the Hunter's Mark spell says, in part (emphasis mine):

[...] Until the spell ends, you deal an extra 1d6 damage to the target whenever you hit it with a weapon attack, [...]

The description of the fighter's Unarmed Fighting fighting style option mentions that (TCoE, p. 42):

At the start of each of your turns, you can deal 1d4 bludgeoning damage to one creature grappled by you.

Is the extra damage from Hunter's Mark added to this bludgeoning damage as well?

As this description never explicitly states that the 1d4 bludgeoning damage comes from a weapon attack, it seems like Hunter's Mark does not apply to this damage.

Answer 1

No, the damage from hunter's mark can't be added to it

An unarmed strike is a melee weapon attack. The Sage Advice Compendium (an official source) makes this clear, in the answer to "What does “melee weapon attack” mean: a melee attack with a weapon or an attack with a melee weapon?" (v2.7, page 13):

For example, an unarmed strike counts as a melee weapon attack, even though the attacker’s body isn’t considered a weapon.

However, even though an unarmed strike itself is a melee weapon attack, the automatic damage during a grapple granted by the Unarmed Fighting option for the Fighting Style feature is not an attack. As stated in the "Making an Attack" section of the combat rules:

If there's ever any question whether something you're doing counts as an attack, the rule is simple: if you're making an attack roll, you're making an attack.

Because the damage done at the start of your turn using the Unarmed Fighting Style does not require an attack roll, it is not an attack at all, so the extra damage from Hunter's Mark does not apply.

Answer 2

The extra damage is added to successful unarmed strike attacks, but not to the grapple's damage.

Per the Sage Advice Compendium's answer to "Can a monk use Stunning Strike with an unarmed strike, even though unarmed strikes aren’t weapons?":

The game often makes exceptions to general rules, and this is an important exception: that unarmed strikes count as melee weapon attacks despite not being weapons.

Hence, even if an unarmed strike does not count as a weapon the attack is still considered as a (melee) weapon attack, fulfilling the requirements for the Hunter's Mark spell.

On the other hand, the damage dealt to a grappled creature at the starting of the turn does not depend on an attack but it is automatically dealt, hence it can not benefit from the Hunter's Mark bonus.

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You can find another part of the SAC which confirms this:

What does “melee weapon attack” mean: a melee attack with a weapon or an attack with a melee weapon?

It means a melee attack with a weapon. Similarly, “ranged weapon attack” means a ranged attack with a weapon. Some attacks count as a melee or ranged weapon attack even if a weapon isn’t involved, as specified in the text of those attacks. For example, an unarmed strike counts as a melee weapon attack, even though the attacker’s body isn’t considered a weapon.

Here’s a bit of wording minutia: we would write “melee-weapon attack” (with a hyphen) if we meant an attack with a melee weapon.

Answer 3

Unfortunately not

The key here is "whenever you hit it with a weapon attack", found in the description of Hunter's Mark.

The effect of Unarmed Fighting says that "you can deal 1d4 bludgeoning damage to one creature grappled by you." You don't roll to hit or anything it just happens.

Therefore, Hunter's Mark will not trigger because you haven't technically hit anything.