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

Instants can be cast at any time you have priority, including during your opponent's draw step. This is the core rule that defines instants as a card type (CR 101.1, 116.1, 304.1). Priority is passed around throughout every step and phase of every player's turn, giving each player the opportunity to cast instants and activate abilities.

During the draw step, the active player draws a card (CR 504.1), then receives priority. After they pass priority (or choose not to act), you as the non-active player receive priority and may cast instants or activate abilities at that point (CR 116.3b).

The key restriction is that sorceries, creatures, enchantments, artifacts, and planeswalkers generally can only be cast during your own main phase when the stack is empty (CR 307.1). Instants have no such restriction — they can be cast during any step or phase, on any player's turn (CR 304.1).

Concrete example: It's your opponent's draw step. They draw their card and pass priority. You cast Counterspell targeting a spell they previously put on the stack — this is completely legal because Counterspell is an instant and you have priority.

Note: Some spells have flash (CR 702.8), which allows non-instant cards to also be cast at instant speed, making them equally flexible in timing.