Are monster attacks magical in 5th edition?

[Glendleton]

A lot of monsters are resistant to non-magical damage from bludgeoning, slashing, and piercing weapons. My question is if a monstrosity, undead, dragon, or something like that hits another that has such resistance, does it apply?

For example, if a Grick (monstrosity) bludgeons a vampire with it's tail, does the vampire only take half damage? What if the vampire punches the Grick?

You could say (and I would agree) that undead and monstrosities are magical creatures, therefore their unarmed attacks count as magic. However, is there a hard rule to settle the issue, or is it subject to interpretation as I suspect?

On a side note, IIRC from AD&D, if a monster has 4+ HD it's attacks are considered at least +1 magic, but I can't find anything like that in 5e.

[Alucard2099]

I believe it will state in the attack of the monster if it magical or not.

[Svata]

I would import a(n updated to fit 5e terminology) rule from 3e/3.5 here. If a creature has resistance to nonmagical weapons, then its natural attacks (but not attacks made with a weapon), are considered magical for the purposes of overcoming resistance to nonmagical weapons.

[Inevitability]

I believe it will state in the attack of the monster if it magical or not.

Alucard is right. So in the case you present above, the grick and the vampire would both deal half damage.

[Kinneus]

Yup. Lots of monsters have "Magic Weapons" as in attribute, with an explanation that their weapon attacks count as magical.

What I'm wondering, though, is if this is meant to imply they're carrying around a bunch of +1s? Like a marilith with six +1 scimitars? Or just that their magical nature imbues the weapon with resistance-piercing magic while they wield it? I'm leaning towards the latter but I can see an argument for the former.

[Zweisteine]

What I'm wondering, though, is if this is meant to imply they're carrying around a bunch of +1s? Like a marilith with six +1 scimitars? Or just that their magical nature imbues the weapon with resistance-piercing magic while they wield it? I'm leaning towards the latter but I can see an argument for the former.

It's the latter. The marilith's magical nature extends to her weapons, giving them enough magic to hurt similarly imbued creatures, but not to actually empower the weapon. Any magical strength the marilith has is accounted for in her actual strength.

Unless otherwise specified, a monster's weapons have no special qualities inherent to themselves.