Union Square station is a light rail station on the Massachusetts Bay Transportation Authority (MBTA) Green Line located in the Union Square neighborhood of southeastern Somerville, Massachusetts. The accessible terminal station has a single island platform serving the two tracks of the Union Square Branch, which parallels the Fitchburg Line. It opened on March 21, 2022, as part of the Green Line Extension (GLX), which added two northern branches to the Green Line.

The Fitchburg Railroad opened a station at Prospect Street in the 1840s; it was renamed to Union Square around 1875. The station was closed in 1938 and demolished around that time. Extensions to the Green Line were proposed throughout the 20th century, but a Union Square spur was not considered until the early 21st century. Several station sites and alignments were considered, with the Prospect Street location and the route along the Fitchburg Line chosen in 2009. The MBTA agreed in 2012 to open the station by 2017, and a construction contract was awarded in 2013.

Cost increases triggered a wholesale reevaluation of the GLX project in 2015. A scaled-down station design was released in 2016, with a design and construction contract issued in 2017. Construction of Union Square station began in early 2020 and was largely completed by late 2021. The station was initially served by the Green Line E branch, but is now served by the Green Line D branch. A major development project is under construction near the station.

Union Square station is located on the east side of the Prospect Street bridge, about 900 feet (270 m) south of the Union Square intersection. The Fitchburg Line runs roughly northwest–southeast through the station area, with the two-track Union Square Branch of the Green Line on the north side of the Fitchburg Line tracks. The station has a single island platform, 225 feet (69 m) long and 20 feet (6.1 m) wide, located about 250 feet (76 m) east of Prospect Street between the Green Line tracks. A canopy covers the full length of the platform.

The platform is 8 inches (200 mm) high for accessible boarding on current light rail vehicles (LRVs), and can be raised to 14 inches (360 mm) for future level boarding with Type 9 and Type 10 LRVs. It is also provisioned for future extension to 300-foot (91 m) length. A starter booth is located at the east end of the platform, with an at-grade emergency exit walkway leading to Allen Street.

The station entrance plaza is located on the east side of the Prospect Street bridge, next to a small utility building. A walkway connects the plaza and platform, with fare vending machines located in a wider section of the walkway at its west end. Tail tracks are located on each side of the walkway. The tail tracks are designed not to preclude further extension to Porter station in the future. To the north of the entrance plaza is "Station Plaza", built by the developer of an adjacent building.The main entrance to the station is a walkway from Prospect Street to the north. An elevator and stairs connect to the Prospect Street bridge.

Public artwork at the station includes Passage by Matthew Trimble – a sculptural archway over the walkway to the platform – as well as murals on station sign panels. Bike racks with space for 34 bikes are located in the entrance plaza. A covered shelter will have racks for 86 bikes. Five MBTA bus routes – 85, 87, 91, 109, and CT2 – stop in Union Square near the station.

The Charlestown Branch Railroad was extended through Somerville in 1841, opening on January 5, 1842. It was absorbed into the Fitchburg Railroad in 1846. Prospect Street station – which was located off Medford Street (now Webster Avenue), slightly west of Prospect Street – was opened within several years to serve the Sand Pit Square area. Like Somerville station and other local stops, Prospect Street was primarily served by Lexington Branch trains in the mid-19th century. The nearby square became Union Square in the 1860s, but the station was not renamed Union Square until around 1875.