Union Grounds, also known as Union Base Ball Park, was a baseball grounds in St. Louis, Missouri. It was home to the St. Louis Maroons of the Union Association during the 1884 season and the Maroons entry in the National League in 1885 and 1886.

Both the Union Association and the St. Louis Maroons were the brainchild of Henry Lucas, and local newspapers often called the venue "the Lucas Park" or just "Lucas Park".

The ballpark was bounded by Jefferson Avenue (west, first base); Howard Street (north, third base); 25th Street (east, left field); and Cass Avenue (south, right field). Mullanphy Street was later cut through what was once right and center fields.

Beginning in 2019, the ballpark site and several adjacent city blocks were cleared off to make room for a National Geospatial-Intelligence Agency facility.

• Retrosheet. "Park Directory". Retrieved 2008-05-23.
• Ballpark information
• Article about St. Louis ballparks, including diagram of Lucas Park

38°38′45″N 90°12′44″W / 38.6459°N 90.2122°W / 38.6459; -90.2122