Can a Creature with Multiattack use it on an Opportunity Attack?

Question

I am familiar with both concepts in D&D 5E — an opportunity attack allows the creature to take one reaction to make an attack; a multiattack allows monsters with the ability to take a number of attacks specified in the description on a single attack action.

But does multiattack apply to opportunity attacks taken on a reaction, or do you just use one of the creature's attacks?

My group is split on this. I think they are able to apply multiattacks, but some of my players disagreed, and frankly, I can see an argument for either way.

Answer 1

No, an opportunity attack is a single melee attack.

From the Player's Basic Rules, page 74:

To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature.

Additionally, multiattack specifies that it can't be used in this way. From the DM Basic Rules, page 6:

A creature that can make multiple attacks on its turn has the Multiattack ability. A creature can’t use Multiattack when making an opportunity attack, which must be a single melee attack.

Multiattack can be used only on the creature's turn, and it explicitly can't be used with opportunity attacks.

Finally, if you look at the statblock of a creature that has Multiattack, you can see that it is listed under the Actions section of the statblock. Multiattack is a specific action that a creature can take, just like a player can take the Attack action or the Use An Object action. Just like a Fighter's Extra Attack, you can't use it every time you make an attack. A Fighter can only use Extra Attack when they take the Attack action, and a creature can only use Multiattack when it takes the Multiattack action.

Answer 2

From the definition:

Multiattack

A creature that can make multiple attacks on its turn has the Multiattack ability. A creature can’t use Multiattack when making an opportunity attack, which must be a single melee attack.

My emphasis.

Answer 3

The only monster that I know of that can use Multiattack as a reaction is the Duergar Hammerer (MToF, p. 188).

Engine of Pain. Once per turn, a creature that attacks the hammerer can target the duergar trapped inside it. The attacker has disadvantage on the attack roll. On a hit the attacker deals an extra 5 (1d10) damage to the hammerer, and the hammerer can respond by using its Multiattack with its reaction.

That is a specific case though, and as other answers have stated, the general rule is no.