If a creature's toughness is reduced below the damage already marked on it, does it die immediately as a state-based action?

Yes — state-based actions handle this automatically and without any player receiving priority. Under CR 704.5g, a creature is destroyed if the total damage marked on it is greater than or equal to its toughness. This check happens continuously whenever a player would receive priority, meaning the moment a creature's toughness drops to or below its marked damage, the game recognizes this and the creature is destroyed before anyone can act.

Toughness reductions can come from many sources: spells, activated abilities, static abilities, or effects like -1/-1 counters. In every case, as soon as the toughness drops to meet the threshold set by the damage already on the creature, state-based actions apply and destroy it. No separate damage event or trigger is needed — the math already exists.

It is worth noting this is distinct from a creature dying to damage itself (CR 704.5g covers both cases). Whether the creature's toughness was reduced to 0 or below, or reduced enough that existing marked damage now equals or exceeds it, the same state-based action fires. The creature does not get a chance to regenerate from the toughness reduction unless regeneration was already 'set up' before the check.

Example: Your 4/4 creature has 3 damage marked on it from combat. Your opponent then casts Shock-like effect targeting it — but instead imagine they activate an ability that gives your creature -1/-1 until end of turn, dropping it to a 3/3. Now its toughness (3) equals its marked damage (3), so state-based actions immediately destroy it the next time they are checked, before anyone gets priority.