Does a -1/-1 counter and a +1/+1 counter on the same creature cancel each other out immediately?

Whenever a permanent has both a +1/+1 counter and a -1/-1 counter on it at the same time, the game automatically removes one of each as a state-based action. This happens before any player can take another action — state-based actions are checked continuously and resolved before players receive priority (CR 704.1, 704.5q).

This annihilation is not a triggered ability and does not use the stack; it simply happens. If a creature somehow had three +1/+1 counters and two -1/-1 counters, the result after state-based actions would be one +1/+1 counter remaining, because one of each pair cancels out until one type is exhausted (CR 704.5q).

This rule matters most when effects like Persist (which returns a creature with a -1/-1 counter) interact with effects like Undying (which returns a creature with a +1/+1 counter), or when proliferate and -1/-1 counter effects overlap on the same creature.

Example: You control a 2/2 creature with one +1/+1 counter. An opponent's spell puts a -1/-1 counter on it. Before anyone gets priority again, state-based actions fire: both counters are removed simultaneously, leaving a plain 2/2 creature with no counters.