Can you cast an instant during your opponent's draw step?

Players receive priority during every step and phase of the game, including the draw step. After your opponent draws their card for the turn, they receive priority first. If they pass priority and you receive it, you may cast instants or activate abilities at instant speed.

The draw step works like any other step: the active player draws (a turn-based action requiring no priority), then the active player receives priority. Once they pass, you get priority and can act (CR 117.3a, CR 504.1).

So yes, casting an instant during the opponent's draw step is legal, as long as you have priority at that moment. Common reasons to do so include responding to something the opponent did, or simply choosing that moment as a strategic window before their main phase.