New York State Route 125 (NY 125) is a 7.50-mile (12.07 km) north–south state highway located within Westchester County, New York, in the United States. The route begins at an intersection with U.S. Route 1 (US 1) in the town of Mamaroneck and ends at a junction with NY 22 in the city of White Plains. A section of the route in the city of White Plains is maintained by Westchester County and co-designated as County Route 26 (CR 26). A second county-owned segment exists along the New Rochelle–Scarsdale line as County Route 129. Both numbers are unsigned. NY 125 was assigned as part of the 1930 renumbering of state highways in New York, initially extending from US 1 to Mamaroneck Avenue in White Plains. It was extended north to NY 22 in the mid-1930s.

NY 125 begins at an intersection with US 1 (Boston Post Road) in the town of Mamaroneck. Proceeding northwest as Weaver Street, NY 125 crosses through a residential section of Mamaroneck as a two-lane local road. It crosses over the New England Thruway (I-95) and the Metro-North Railroad's New Haven Line (shared with Amtrak as its Northeast Corridor) before bending southward around a school and climbing into the residential neighborhoods overlooking Mamaroneck. Passing Bonnie Briar Country Club, the route crosses the Sheldrake River and turns northward to run along the border between the city of New Rochelle and the village of Scarsdale. After exiting Mamaroneck, the route becomes county-maintained and co-designated as CR 129.

It briefly runs along the eastern edge of New Rochelle's Pinebrook Heights and Pinebrook neighborhoods ahead of an interchange with the Hutchinson River Parkway (exit 20) in the Quaker Ridge section of New Rochelle. County maintenance ends just south of the junction while NY 125 continues past the parkway along the New Rochelle–Scarsdale line. It serves residential parts of both communities prior to fully entering Scarsdale and accessing the Heathcote section of the village. Here, the route connects to the west end of the Heathcote Bypass, a two-lane divided highway running below grade along the northern edge of the neighborhood. NY 125 continues directly into the center of Heathcote, where it turns northeast onto Palmer Avenue at an intersection known locally as the Five Corners. The route continues northeastward for several blocks before gradually turning back to the north as it heads into nearby Secor Gardens.

The Palmer Avenue name ends at a junction with Secor Road, at which point NY 125 becomes known by the latter name. One block to the north, the highway connects to the east end of the Heathcote Bypass. From here, NY 125 takes a northeasterly track out of Secor Gardens and Scarsdale, passing the Fenway Golf Club on its way into the city of White Plains. Across the city line, maintenance of the route shifts to the county once again as CR 26. Now known as Old Mamaroneck Road, NY 125 serves several of the city's residential neighborhoods as it approaches downtown White Plains. The highway crosses into Gedney Park, becoming a two-lane residential street as it approaches Mamaroneck Avenue. At this junction, NY 125 shifts from a county-maintained road to a city street as it turns northwest through downtown White Plains. The route continues for another three blocks to its north end at junction with NY 22 (East Post Road), several blocks south of NY 119 (Main Street).

On July 23, 1900, the state of New York awarded a contract to rebuild 2.78 miles (4.47 km) of roads in Mamaroneck and Scarsdale. The project covered the northernmost half-mile (0.8 km) and all but the northernmost 0.3 miles (0.48 km) of what is now NY 125 in Mamaroneck and Scarsdale, respectively. Both roads were added to the state highway system on October 24, 1901, as unsigned State Highway 19 (SH 19). The remainder of the route in Mamaroneck was added in the mid-1920s, and the 0.3-mile (0.48 km) segment leading south from the White Plains city line was taken over by the state sometime after 1926. NY 125 was assigned as part of the 1930 renumbering of state highways in New York. It began at US 1 and went north over the state highways in Mamaroneck and Scarsdale and locally maintained streets in New Rochelle and White Plains to Mamaroneck Avenue, then NY 126. The NY 126 designation was removed c. 1936, at which time NY 125 was extended north to NY 22.

In the mid-20th century, the route was proposed to be realigned onto an expressway loosely following the Bronx River Parkway and NY 22 between the Bronx and White Plains. While the parkway was restricted to non-commercial vehicles, the proposed highway was intended to serve vehicles of all types. The proposal was abandoned in the 1960s. Part of the expressway's planned route along the former right-of-way of the New York, Westchester and Boston Railway's line to White Plains was eventually used for the Heathcote Bypass in the mid-1960s.

The entire route is in Westchester County.