Chelsea Waterside Park, formerly Thomas F. Smith Park, is a public park located at West 23rd Street between 11th and 12th Avenues along the West Side Highway in Chelsea, Manhattan, New York City. It was originally operated by the government of New York City under the New York City Department of Parks and Recreation. As of 2023^[update] it is part of the Chelsea section of Hudson River Park and managed by the Hudson River Park Trust.

The park was originally the site of a small freight yard for the Erie Railroad. In 1906, the railroad redeveloped the site into a park, as part of the reconstruction of the adjacent ferry terminal. In 1915, the park was taken over by the Parks Department, and was named for politician Thomas Francis Smith following his death in 1923. The construction of the West Side Elevated Highway in the early 1930s split the park into two adjacent sections.

Chelsea Waterside Park was designed in the late 1980s by architect Thomas Balsley. Half of the proposed park would be an expansion of the existing Smith Park, and the other half would be developed on the waterfront atop Piers 62, 63 and 64, with the two halves connected by a footbridge. The inland portion of Chelsea Waterside Park was constructed in the 1990s as a part of Hudson River Park during the redevelopment of the West Side Highway, and opened in 2000. The waterfront sections proposed for the park were completed in 2010 under a separate project. Between 2017 and 2023, major renovations took place in Chelsea Waterside Park, with a redesigned playground opening in 2018 and the remaining upgrades completed by 2023.

Chelsea Waterside Park is located on a triangular two-block long site at the west end of 23rd Street. It is bound to the west by 12th Avenue (West Side Highway), to the east by 11th Avenue, to the south by West 22nd Street, and to the north by West 24th Street. The de-mapped western end of 23rd Street runs through the park as a pedestrian plaza, connecting to the rest of Hudson River Park via a crosswalk. The current park is 2.5 acres (1.0 ha) in size. The portion of the park south of 23rd Street formed the original Thomas F. Smith Park, which had an area of between 0.5 and 2 acres (0.20 and 0.81 ha).

Located to the west of the park is the main right-of-way of Hudson River Park, which runs north-to-south between the West Side Highway and the Hudson River. Immediately across from Chelsea Waterside Park are the Chelsea Piers sports complex, and Piers 62, 63, and 64. Across 24th Street to the north is the United States Postal Service Manhattan Vehicles Maintenance Facility. To the east at 10th Avenue is the High Line. The park is located just south of the West Chelsea Historic District, which includes the landmarked Starrett–Lehigh Building. The park is located in the vicinity of the Chelsea-Elliot Houses and Fulton Houses housing projects, and the Penn South and London Terrace apartment complexes.

The 2000-built design of Chelsea Waterside Park was created by landscape architect Thomas Balsley during the reconstruction of the West Side Highway in the late 1980s and 1990s. Balsley would later design renovations to Gantry Plaza State Park, Riverside Park South, and the East River Esplanade. The park was reconstructed between 2017 and 2023, with Phase I of the renovations designed by Michael Van Valkenburgh Associates, and Phase II designed by the Abel Bainnson Butz (ABB) and CDR Studio Architects firms. As designed by Balsley, Chelsea Waterside Park contains walls and benches constructed of granite in order to resemble 19th-century coastal bulkheads, a reference to the area's historical use as a marine port.

The pedestrian plaza along 23rd Street was originally constructed as a straight-path promenade between 11th and 12th Avenues, intended as a gateway to the main Hudson River Park along the river. The Phase II renovations by ABB reimagined the promenade as the "central area", replacing the straight path with two curved paths surrounding a central lawn. Within the lawn is a picnic area with "festive lights" hanging above for nighttime use. In Balsley's initial 1980s park plans, a landscaped or terraced footbridge called a "platform park" would have run above the highway from the pedestrian plaza towards the shorefront sections of Hudson River Park.

At the north end of the park are an artificial turf athletic field used for soccer and other sports, and a basketball court. The turf field measures 32 yards (29 m) by 60 yards (55 m). Both the athletic field and the basketball court have lights for nighttime use. Next to the athletic field at the northwest corner of the park is a smaller elevated sitting area known as the "sunset overlook".