If a creature's toughness is reduced to 0 by a continuous effect without damage, does it die from state-based actions?

State-based actions are checked continuously by the game whenever a player would receive priority. One of those actions, defined in CR 704.5f, states: if a creature has toughness 0 or less, it is put into its owner's graveyard. This applies no matter how the toughness reached 0 — damage is not required.

Continuous effects that modify toughness (such as those from Auras, Equipment, counters, or spells like Marshaling Cry or Gloom Surgeon effects) can reduce a creature's toughness. If any such effect brings the value to 0 or less, the creature dies the next time state-based actions are checked, before any player can take another action.

It is important to distinguish this from lethal damage (CR 704.5g), which requires a creature to have damage marked on it equal to or greater than its toughness. Toughness-reducing effects bypass that entirely — there is no damage involved, yet the creature still dies via 704.5f.

Concrete example: Your opponent controls a 2/2 Bear. You cast Diminish (which sets a creature's base power and toughness to 1/1 until end of turn) and follow it up with Sudden Spoiling (which sets all creatures' base power and toughness to 0/2)... but simpler: if you enchant the 2/2 with an Aura that gives -2/-2, it becomes a 0/0. The next time state-based actions are checked (immediately, before anyone gets priority), the creature is put into the graveyard under CR 704.5f — no damage needed.

Note that regeneration and indestructible do not save a creature from dying this way; those abilities only protect against destruction and lethal damage, not against toughness dropping to 0 (CR 702.12b, 702.15).