The Upper West Side main broke on Broadway, between West 105th and West 106th Streets, just before 6:30 a.m.
Water quickly flooded the sidewalk and several basements, and also closed two lanes of Broadway to traffic.
A separate water main break flooded First Avenue on the Upper East Side just after 1 a.m.
That 12-inch main, which dates back to 1917, flooded First Avenue at East 62nd Street.
Several buildings on First Avenue had flooded basements, and DEP workers responded to shut off the main.
First Avenue was expected to be partially closed for the morning commute.