Can you grapple a creature as an opportunity attack?

Question

This came up in a game I was in the other day, so I'm curious as to know how this works.

A gnome was attempting to flee past us up the stairs and I declared that I'd like to try and stop him, by using my reaction to make a grapple - opportunity attack. My DM declared that I wasn't able to use grapple as an Opportunity Attack (obviously what the DM says is fine, but I want to know what RAW/RAI says). Instead I had to "attack" the gnome with non-lethal force (which scared the gnome into running more).

Can you grapple as an Opportunity Attack?

Grappling:

When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. If you’re able to make multiple attacks with the Attack action, this attack replaces one of them. (PHB pg.195)

Opportunity Attack:

You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack interrupts the provoking creature’s movement, occurring right before the creature leaves your reach. (ibid)

My thoughts on this are that

• RAW: You need to use the Attack action to make a grapple.
• RAI: Grapple is a melee attack and can be used as an opportunity attack.

Answer 1

As Miniman's answer points out, you cannot grapple as an opportunity attack because an opportunity attack does not give you an Attack action. However, your situation doesn't actually call for an opportunity attack. Instead, it sounds like you had readied an action. From Basic Rules page 72:

Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn so that you can act later in the round using your reaction.

First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it...

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.

When you ready an action, you declare that you intend to do something out of your turn in response to something specific happening. For example, you can ready an action by saying "If an enemy comes through that door, I will fire an arrow at them." If an enemy does come through the door, you can then immediately use the readied action and shoot them.

Therefore, if you state on your turn, "I want to grapple the gnome if he tries to get past me," and you have not taken another action on your turn, you would be able to grapple the gnome as he moves past you. If the gnome doesn't try to get by you, the action is wasted. You can also choose to ignore the trigger and take an attack of opportunity as your reaction instead.

Keep in mind that you must declare a Ready action on your turn. Ready actions are actions, like attacks or casting spells. If your turn had already passed when you declared you'd grapple the gnome, your DM would be right in ruling that you can only make an attack. However, if you declared it on your turn, and hadn't already attacked or acted, your DM should have allowed you to grab the gnome.

Answer 2

You've already quoted the important distinction. A grapple requires you to use the Attack action, which an opportunity attack does not give you. Your DM is free to rule that you can do this, but they have already ruled that you can't, so that seems unlikely.

As far as RAI are concerned, we can only speculate, but the writers of 5e have been extremely careful about things requiring an Attack action vs an attack. It doesn't seem at all probable that they meant for grappling to merely require a melee attack and accidentally wrote that it required the Attack action.

Answer 3

Though the ability to do as you wished is available. it requires a character to devote the time and training necessary to be ready for such a situation. This level of muscle memory is developed by taking the SENTINEL feat

Answer 4

One solution:

On your turn, take an attack action, declare you "want to grab a creature or wrestle with it" this allows you to make special melee attacks called grapples. Now your melee attacks are grapples if you replace the melee attack with the grapple. Since it does not declare you can grapple only once per turn, nothing prevents you from making each of your melee attack as a grapple.

Make a grapple attack, if you miss, you can now can use your bonus actions and reactions to grapple instead of make a melee attack since you choose the Attack action and declared you "want to grab a creature or wrestle with it" which enables the special melee attacks.

TLDR: As long as your last action was an Attack action and you "want" to grapple during that action, you are able to make grapples instead of melee attacks for your attack actions, bonus actions, and reactions.