| Box Width: | 300px |
| Subheader: | Houston Street Crosstown |
| System: | MTA Regional Bus Operations |
| Operator: | New York City Transit Authority |
| Garage: | Michael J. Quill Depot |
| Vehicle: | New Flyer Xcelsior XD40 New Flyer Xcelsior XE40 Nova Bus LFS HEV |
| Locale: | Manhattan, New York, U.S. |
| Communities: | West Village, SoHo, Lower East Side |
| Start: | West Village – Spring St and Hudson St West Village - 6th Avenue and Spring Street (PM rush hour) |
| Via: | Houston St |
| End: | Lower East Side – Grand Street and FDR Drive |
| Length: | 2.5miles |
| Alt Frequency: | Every 30 minutes |
| Day: | 6:30 AM-11:50 PM |
| Timetable Link: | M21 |
| Annualpatronage: | 300,809 (2023)[1] |
| Transfers: | Yes |
| Map State: | collapsed |
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. The M9 and M21 are operated by the New York City Transit Authority, and based out of the Michael J. Quill Depot.
The M21 bus route begins at Spring Street and Hudson Street in the West Village. Eastbound buses use Sixth Avenue to access Houston Street, whereas westbound buses use Washington Street to access the terminus. The M21 stays on Houston Street until Columbia Street. Eastbound buses continue to FDR Drive, and use the service road to access Grand Street, where they terminate. Westbound buses then use Lewis Street, Delancey Street, and Columbia Street to access Houston St, where they head back to the West Side.[2] During the afternoon rush hour, M21 buses short turn at Sixth Avenue instead of terminating at Washington Street.
The M21 was originally the New York City Department of Plant and Structures bus route M10, and later became New York City Omnibus Corporation's route 21. This route replaced New York Railways' Avenue C Line streetcar on September 21, 1919.
Due to the 2010 budget crisis, the M21 which ran to 29th Street via Avenue C and weekend service was discontinued on June 27, 2010, in conjunction with an extension to its current terminus at Grand Street and FDR Drive.[3] Weekend service was restored on January 6, 2013.[4] On January 3, 2022, the M21 had its western terminus during PM rush hours truncated to 6th Avenue due to rush hour traffic on Varick Street approaching the Holland Tunnel.[5] [6]