In a multiplayer game, can you deal combat damage to a player who has already lost but whose permanents are still on the battlefield?

When a player loses the game in a multiplayer game, they are removed from the game immediately as a state-based action or triggered effect resolves. Under CR 800.4a, when a player leaves the game, all objects that player owns leave the game as well, and all spells and abilities they control cease to exist. This happens before any subsequent combat damage step could apply to them.

The key point is timing: a player's removal is processed as soon as the loss condition is met (CR 104.3, CR 800.4). If a player is eliminated between the declare attackers step and the combat damage step — for example, because a triggered ability drained their last life — the attacked player no longer exists as a legal target or recipient of damage. CR 510.1 requires combat damage to be assigned to defending players or planeswalkers they control, but a removed player is no longer a 'defending player.'

Additionally, CR 800.4a specifies that permanents owned by the leaving player leave the battlefield immediately. So those permanents are gone before the combat damage step would resolve — there is nothing left to interact with in combat.

Concrete example: You attack Player B with a 5/5. Before combat damage is dealt, Player C's drain effect kills Player B (reducing them to 0 life). Player B is immediately eliminated and removed. Your 5/5's combat damage step occurs, but Player B no longer exists as a recipient — the damage is simply not dealt to anyone (unless you can redirect, which requires a legal target).

This also means that if you declared an attack against a player who then loses mid-combat, any creatures that were attacking that player do not automatically redirect their attacks — they simply lose their attack target and the damage goes nowhere, per CR 800.4d.