- Southern Terminus:
- Montauk Highway (CR 80) in Brookhaven
- Northern Terminus:
- NY 25A in Rocky Point
- 11.75 miles
CR 21 is a north/south route crossing Long Island in the vicinity of Yaphank. The southernmost section is Yaphank Avenue, which extends from Montauk Highway (CR 80) in Brookhaven to just north of the Long Island Expressway (I-495) in Yaphank. Yaphank Avenue was originally continuous, but it was cut into two sections by the construction of Sunrise Highway (NY 27). No overpass was built because motorists wishing to cross the highway can use Horseblock Road (CR 16) just to the east. The southern section extends just a few blocks from the ramp for exit 57S to Montauk Highway (CR 80).
At the northern end of Yaphank Avenue, CR 21 turns west onto Main Street in Yaphank from about half a mile and then north onto Yaphank-Middle Island Road. At Middle Country Road (NY 25), Yaphank-Middle Island Road turns into Rocky Point Road. CR 21 continues north to NY 25A in Rocky Point, where it ends.