Does a creature with undying return to the battlefield if it had a +1/+1 counter when it died?

Undying reads: 'When this creature dies, if it had no +1/+1 counters on it, return it to the battlefield under its owner's control with a +1/+1 counter on it.' The condition is explicit — the creature must have had no +1/+1 counters at the moment it died for undying to trigger at all.

This is governed by CR 702.92a, which defines undying, and the trigger checks the game state at the time of the creature's death (the last-known information principle, CR 702.92a and CR 603.4). If even a single +1/+1 counter was on the creature when it moved to the graveyard, the trigger condition is not met and undying simply does nothing.

It is important to note that -1/-1 counters interact differently: if a creature with undying has both a +1/+1 counter and a -1/-1 counter, state-based actions (CR 704.5q) will remove one of each, potentially leaving the creature with no +1/+1 counters — in which case undying would trigger if it then dies.

Concrete example: You control a Strangleroot Geist (which has undying) that has received a +1/+1 counter from a previous undying trigger. Your opponent destroys it. Because it had a +1/+1 counter on it when it died, undying does not trigger, and Strangleroot Geist stays in the graveyard.