The 20th Assembly district of Wisconsin is one of 99 districts in the Wisconsin State Assembly. Located in southeastern Wisconsin, the district is entirely contained in southeastern Milwaukee County. It comprises the suburban cities of Cudahy, South Milwaukee, and St. Francis, and some of the far south side of the city of Milwaukee including the Fernwood neighborhood and part of the Bay View neighborhood. The district also contains Saint Francis de Sales Seminary, Sheridan Park, and about half of the Oak Creek Parkway. The district is represented by Democrat Christine Sinicki, since January 1999.

The 20th Assembly district is located within Wisconsin's 7th Senate district, along with the 19th and 21st Assembly districts.