If you return a land to your hand with Unsummon during your opponent's turn, can you replay it on your next turn?

Unsummon returns a land to its owner's hand. Once a land is in your hand, it is simply a card in your hand — it has no memory of having been on the battlefield. There is no rule that prevents you from playing a land that was previously on the battlefield.

The key rule is CR 305.2: a player may play one land per turn during their main phase, and only when the stack is empty. This limit is tracked per turn, not per land card. So if you have not yet played a land on your next turn, you may play the returned land as your one land drop for that turn.

CR 701.22a confirms that 'playing a land' means putting it onto the battlefield from your hand during your turn. The land being bounced by Unsummon on your opponent's turn does not consume your land drop for your next turn — those are entirely separate turns.

Concrete example: Your opponent controls a land you enchanted with Spreading Seas. On their turn, you play Unsummon on that land, returning it to their hand. On their next turn, they may replay it as their one land drop for that turn, now free of the enchantment.

Note: If you have already played a land this turn before Unsummon resolves, you cannot play the returned land again this same turn (CR 305.2). You would have to wait until your next turn.