The West Kill, an 11-mile-long (18 km) tributary of Schoharie Creek, flows through the town of Lexington, New York, United States, from its source on Hunter Mountain, the second-highest peak of the Catskill Mountains. Ultimately its waters reach the Hudson River via the Mohawk. Since it drains into the Schoharie upstream of Schoharie Reservoir, it is part of the New York City water supply system. It lends its name to both a mountain to its south and a small town midway along its length.

The West Kill's 31.2-square-mile (81 km²) watershed accounts for 10 percent of the reservoir's basin. It has the highest elevations and steepest slopes of any of the Schoharie's subwatersheds, with runoff from seven of the 35 Catskill High Peaks draining into the stream. Due to limited development and extensive land protection in the stream's watershed, its water is relatively clean, supporting a habitat for both wild and stocked trout; historically it has drawn fly fishers and other anglers. However, the West Kill has contributed to turbidity issues with the Schoharie creek and reservoir due to recent floods; several government agencies have worked together to develop a management plan that will mitigate the floods and the turbidity.

The upper 8 miles (13 km) of the West Kill flows west through the Spruceton Valley to the hamlet of West Kill. From there it turns to a more northerly course to the Schoharie at Lexington.

Two streams that later join rise in the cirque between Hunter and Southwest Hunter mountains, amidst the dense forests of the West Kill Wilderness Area, part of the Catskill Park. The source of the northern stream is at 3,100 feet (940 m), the higher of the two. It flows through a narrow groove down the steep upper slopes of the cirque for its first quarter-mile (400 m).

Just under 2,700 feet (820 m) in elevation, the terrain becomes gentler. At the town line between Hunter and Lexington, the two streams join. The West Kill flows steadily downhill for its next half-mile (800 m) as the Devil's Path hiking trail, itself descending the mountain, gradually comes closer to the stream and follows it along its north side.

At Diamond Notch Falls, the Devil's Path merges briefly with the Diamond Notch Trail coming in from the west. The two cross the West Kill on a wooden bridge, the uppermost crossing of the stream. Just south of the stream, the trails again diverge, with the Devil's Path following the stream for a short distance on that side before beginning its ascent of West Kill Mountain to the southwest. The Diamond Notch Trail runs parallel to the kill for another 0.7 miles (1.1 km) to the trailhead parking lot, the eastern end of Greene County Route 6, known locally as Spruceton Road, at 2,220 feet (680 m) elevation.

Shortly after that, the valley begins to widen slightly. The West Kill receives its first tributary, an unnamed stream that flows into it from the slopes of the eponymous mountain to its south. West of that confluence the kill begins to pass some cleared areas and structures. As Spruceton Road bends to the north away from the stream, its first named tributary, Hunter Brook, flows in from the north just 500 feet (150 m) east of where Spruceton Road crosses. After receiving Pettit Brook from the south, Spruceton Road returns to the north side of West Kill.

Privately owned Wolff Road crosses the West Kill 2,500 feet (760 m) beyond. A half-mile further west, a short local street, Ad Van Road, crosses. Just below, at the former hamlet of Spruceton, Herdman Brook flows into the West Kill from the slopes of Evergreen Mountain to the north. Styles Brook follows shortly, draining the cirque below West Kill Mountain's summit, from the south, just west of where Baker Road crosses to provide access to several farms on that side. Cleared fields and structures are now found on both sides of the stream.