New York State Route 121 (NY 121) is a north–south state highway in the Hudson Valley of New York, United States. It begins in northern Westchester County at an intersection with NY 22 in Bedford and extends for 15.56 miles (25.04 km) to a junction with U.S. Route 6 and U.S. Route 202 (US 6 and US 202) east of the village of Brewster in Putnam County. As the route heads north, it briefly overlaps with NY 35 and NY 116 in Westchester County and connects to Interstate 84 (I-84) in Putnam County. NY 121 was assigned as part of the 1930 renumbering of state highways in New York.

NY 121 begins at a junction with NY 22 (Cantitoe Street) in the hamlet of Bedford Village, just north of the junction with NY 172. The route proceeds northeast out of Bedford Village, crossing through the town of Bedford and over the Stone Hill River. Winding northeast and soon north through the dense woodlands of Bedford, NY 121 reaches a junction with the northern terminus of NY 137 (Stone Hill Road). The route winds north through Bedford as a two-lane road past Pea Pond before reaching the southern end of the Cross River Reservoir. Just after the junction with Upper Hook Road, NY 121 crosses over the southern extremities of the reservoir, crossing into the town of Pound Ridge.

Now in Pound Ridge, NY 121 runs along a small branch of the of Cross River Reservoir, crossing into the town of Lewisboro and gaining the moniker of Cross River Road. The route soon enters the hamlet of Cross River, where it meets the namesake river, reaching a junction with NY 35. NY 35 and NY 121 become concurrent, crossing northeast through the hamlet before reaching Cross River Plaza, where the routes fork. Leaving Cross River, NY 121 continues north through Lewisboro, returning to the dense woodlands. After passing west of Waccabuc, the route reaches a junction with the terminus of NY 138 just before crossing the town line from Lewisboro into North Salem.

NY 121 winds northeast through North Salem, gaining the moniker of Grant Road, soon reaching the hamlet of Grant Corner. Here, the route intersects with June Road (unsigned County Route 310 or CR 310). Beginning a parallel with the Titicus River, NY 121 soon reaches a junction with NY 116, which become concurrent as NY 121 bypasses the hamlet of North Salem. At the junction with Peach Lake Road, NY 116 continues east on Titicus Road while NY 121 turns north on Peach Lake. Continuing north through North Salem, NY 121 begins running to the east of Peach Lake, retaining the Peach Lake Road name until reaching the Putnam County line in the hamlet of Peach Lake.

Now in the town of Southeast, NY 121 runs alongside the Vail's Grove Golf Course and soon into exit 21, a pair of ramps from I-84. Just north of the I-84 interchange, NY 121 terminates at an intersection with US 6 and US 202 in front of the East Branch Reservoir.

The section of NY 121 in Westchester County was first improved to state highway standards as part of three separate projects contracted out by the state of New York in 1903. South of NY 35, the road was rebuilt under a contract awarded on July 19 and added to the state highway system on June 12, 1905, as part of State Highway 144 (SH 144). The remainder of the route in the county was reconstructed as part of contracts let on June 14 and June 19, with the former covering the segment between the west end of the NY 35 overlap and NY 138. The roads improved by both projects were accepted into the state highway system on December 21 and 26, 1907, as SH 150 and SH 151, respectively. All three state highway numbers are unsigned. In Putnam County, modern NY 121 was locally maintained until the latter half of the 1920s. NY 121 was designated in the 1930 renumbering of state highways in New York to its current alignment.