The M21 bus route constitutes a public transit line in Manhattan, New York City.

The M21 operates between the West Village and the Lower East Side, serving as a Houston Street crosstown.

Eastbound buses continue to FDR Drive, and use the service road to access Grand Street, where they terminate.

[2][3] During the afternoon rush hour, M21 buses short turn at Sixth Avenue instead of terminating at Washington Street.

This route replaced New York Railways' Avenue C Line streetcar on September 21, 1919.