Does giving a creature -1/-1 counters remove +1/+1 counters on it first?

When a creature has both +1/+1 counters and -1/-1 counters on it at the same time, state-based actions (SBAs) kick in and remove one counter of each type simultaneously, repeating until only one type remains. This is governed by CR 704.5q.

Importantly, this is not a targeted removal — it is an automatic game rule that applies whenever the game checks SBAs, which happens continuously (before each player would receive priority). You cannot choose to 'keep' both types of counters on a creature.

So in practical terms, putting -1/-1 counters on a creature that already has +1/+1 counters effectively cancels them out one-for-one. A creature with three +1/+1 counters that receives two -1/-1 counters will end up with exactly one +1/+1 counter and zero -1/-1 counters after SBAs apply.

Example: Your Strangleroot Geist has two +1/+1 counters on it. Your opponent activates an ability that places three -1/-1 counters on it. After SBAs are checked, the two +1/+1 counters and two of the three -1/-1 counters annihilate each other, leaving the Geist with one -1/-1 counter and zero +1/+1 counters.

This rule exists to keep the game clean and consistent, preventing a creature from simultaneously being boosted and weakened by stacking opposite counter types indefinitely.