If a creature with menace is only blocked by one creature, is that block legal?

Menace is an evasion ability defined in CR 702.110a: a creature with menace can't be blocked except by two or more creatures. This means the defending player must assign at least two creatures to block the menace creature, or leave it unblocked entirely.

During the declare blockers step (CR 509.1), the game checks that all blocking declarations are legal. If a defending player attempts to block a menace creature with only one creature, that declaration is illegal and cannot stand — the game will require the player to make a legal choice instead (CR 509.1b).

Note that the requirement is checked at the moment blockers are declared. If a creature gains menace after blockers are declared, the existing block remains legal — but at the time of declaration, two or more blockers are mandatory.

Example: Your opponent attacks with a 3/3 that has menace. You have a 2/2 and a 1/1. You may block with both creatures (legal), block with neither (legal), but you may NOT block with only the 2/2 — that single-blocker declaration is illegal under the menace rules.