Do tiny creatures threaten the square they are in for attacks of opportunity?

Hey everyone. I know that tiny creatures have a reach of 0ft and so have to enter the square of the creature that they want to attack, thus provoking an attack of opportunity. If a medium creature has a tiny creature in its square after an attack and wants to move away, will this provoke an AoO from the tiny creature, or does its 0ft reach essentially mean it doesn't even threaten the square it's in?

You threaten all squares you can attack. Tiny creatures can attack things in their own square. Therefore, yes, it threatens and the Medium creature provokes for leaving the square.

Separately, you also provoke an AoO for moving into a square occupied by another creature (unless you make an Acrobatics check against their CMD). So if the Medium creature tried to move through the square of a Tiny creature, it would potentially provoke twice- once for entering the square and once for leaving.

Yup.

Threatened Squares

You threaten all squares into which you can make a melee attack, even when it is not your turn.

I can't find any rules to support the idea that entering the square of a tiny creature provokes an attack of opportunity. You are not leaving a threatened square as you enter. You are also allowed to occupy a square with a tiny creature, unlike a medium or small creature. You are not required to do a combat maneuver to enter, so I can't see why it would provoke.

Here's your rules right here.

A Fine, Diminutive, or Tiny creature can move into or through an occupied square. The creature provokes attacks of opportunity when doing so.

...

A big creature can move through a square occupied by a creature three size categories smaller than it is. Creatures moving through squares occupied by other creatures provoke attacks of opportunity from those creatures.

This says you unequivocally provoke for moving through the square of a smaller creature. One could conceivably argue this is merely a restatement of the general rule about leaving threatened squares, but I find that reading unnecessarily tortured- it seems pretty clear to me that it is establishing this as a unique and separate provoking action.

That’s great to hear. Thanks for the help all!

So if the Medium creature tried to move through the square of a Tiny creature, it would potentially provoke twice- once for entering the square and once for leaving.

I don't think it would. There was an FAQ related to this:

In the section on Tiny and smaller creatures, it says that entering a creature’s space provokes an attack of opportunity, but typically 5-foot steps don’t provoke an attack of opportunity. If a Tiny or smaller creature took a 5-foot step into a creature’s space, would it provoke an attack of opportunity?

Yes. Even with a 5-foot step, a Tiny or smaller creature entering a creature’s space provokes an attack of opportunity (unless it is using a more specific ability to avoid the attack of opportunity such as the Monkey Shine feat). This doesn’t mean that a Tiny or smaller creature entering a creature’s space and moving out of a threatened square with a move action provokes two attacks of opportunity from that creature, for the same reason that moving out of multiple of a creature’s threatened squares in the same move action doesn’t provoke two attacks of opportunity.

I find it all fairly counter-intuitive so I couldn't say if a medium creature moving into a Tiny creature's space provokes or not; the "can move into or through an occupied square" quote above suggests to me they're considered separate types of move.

That’s great to hear. Thanks for the help all!

Glad we could help!

[FAQ stuff]

I wasn't aware of that FAQ, and I'm not really sure how to interpret it now. Huh.

This doesn’t mean that a Tiny or smaller creature entering a creature’s space and moving out of a threatened square with a move action provokes two attacks of opportunity from that creature, for the same reason that moving out of multiple of a creature’s threatened squares in the same move action doesn’t provoke two attacks of opportunity.

Huh... This is a very strange thing for them to say when they're talking about a small or larger sized creature. What's the point in even discussing if moving into the square provokes if the threatened area is going to be larger than their square and leaving that area will provoke anyway?

In fact there's hardly reason to be mentioning this (entering a square provoking an AoO) at all, since it seems to be saying that moving through a creatures square [doesn't] provoke an AoO, because it's impossible for something that can attack to not be threatening it's own square. Meaning the only case the two AoOs apply is when one ends their turn on the opponent's square, and the opponent is tiny or smaller (or otherwise has no additional reach), since that is the only situation where entering a square could effectively provoke an AoO.

Because they don't word it like that, it is a very dubious clarification/rule on their part if you ask me.

They were primarily talking about Tiny creatures moving into the square of larger creatures; it wasn't clear whether you could avoid the AoO for that by taking a 5' step. (Indeed, it wasn't clear if it was a rule, or just a reminder that for a tiny creature to move into a medium creature's space they'd normally be leaving a threatened square.) And their answer was, it is a separate rule and it takes priority over 5 foot steps.

I consider any rule about moving 'through' a creature's space to have no clear interpretation. To me 'through' implies either moving into and then out of their space in the same turn or moving from one space occupied by a large creature to another space also occupied by that same creature. For any other circumstance I'd want it to say something like 'moving into' or 'moving out of' for clarity.

In fact there's hardly reason to be mentioning this (entering a square provoking an AoO) at all, since it seems to be saying that moving through a creatures square [doesn't] provoke an AoO, because it's impossible for something that can attack to not be threatening it's own square.

What about someone with a reach weapon?

Also, when a creature is moving into your square, isn't it the square that the creature is leaving that you'd have to threaten? Normally you take the AoO before the move, not after.

What about someone with a reach weapon?

That wouldn't matter; since if they can't provoke those squares in the first place there's no point to say that they won't get an AoO.

when a creature is moving into your square, isn't it the square that the creature is leaving that you'd have to threaten

Yes, but I'm talking about moving through an opponent's square. The FAQ talked about how moving through an opponents square won't trigger an additional AoO despite the fact that there's those two separate actions of leaving a threatened square and crossing the opponent's square.

I suppose one could also say that it might not even count as crossing an opponent's square if they're ending their turn on that square, in which case the FAQ clarification is 100% pointless as far as I can tell.

There'd be no way to cross over a target that can provoke it's own square without provoking normally (leaving a threatened area). I guess alternatively one can say that the rule "moving through another creature's square provokes an AoO" is 100% useless instead.

Either way it's strange.