Can you return a card to your hand from the graveyard during an opponent's turn with a flash spell?

Flash (CR 702.8) allows a spell or ability to be cast or activated at any time you could cast an instant — including during an opponent's turn, at instant speed. This means if a card that returns something from your graveyard to your hand has flash or is itself an instant, you can absolutely use it outside your own turn.

The key distinction is what kind of effect you are using. An instant-speed spell (like a card with flash or one that is simply an instant) can be cast during an opponent's turn as long as you have priority (CR 116.1). Activated abilities with no timing restriction also default to instant speed under CR 509.2 unless they specify otherwise (e.g., 'activate only as a sorcery').

So there are two common ways to do this legally: (1) cast an instant or flash spell that instructs you to return a card from your graveyard to your hand, or (2) activate an activated ability at instant speed that does the same. Sorcery-speed effects or abilities that say 'activate only as a sorcery' cannot be used on an opponent's turn.

Example: Your opponent attacks you. In response, you cast Regrowth... wait, Regrowth is a sorcery. Instead, imagine you cast a flash spell like a hypothetical 'Reclaim (Flash — Return target card from your graveyard to your hand)' during their combat phase. Because it has flash, you cast it at instant speed on their turn, retrieve a card from your graveyard, and it resolves normally.

In short: the ability to act on an opponent's turn comes from flash or the effect being an instant (or instant-speed activated ability), not from the graveyard interaction itself. Always check the card's type line and any timing restrictions (CR 702.8, CR 116.1, CR 509.2).