New York State Route 100C (NY 100C) is an east–west spur route of NY 100 located in Westchester County, New York, in the United States.

NY 100C begins as Old Saw Mill River Road at an at-grade intersection with NY 9A (Saw Mill River Road) in the town of Greenburgh.

After Old Saw Mill River Road leaves it at a roundabout, the route heads east as Grasslands Road and crosses over NY 9A, where it has a northbound entrance ramp.

Continuing east, NY 100C enters a grade-separated interchange with the Sprain Brook Parkway.

[3] In the late 1940s, NY 9A was extended northward through Hawthorne to Ossining by way of a realigned Saw Mill River Road.