Union Square is a skyscraper complex at Sixth Avenue between Union and University Streets in Downtown Seattle, Washington, adjacent to Freeway Park. It consists of two skyscrapers built in the 1980s and primarily used for office space. The entire complex features a 1,100-stall parking garage, a courtyard, a retail plaza spanning three stories and an underground pedestrian concourse that connects with the Fifth Avenue Theater and Rainier Square. Both structures were awarded LEED certification in 2009 and eventually received LEED Platinum certification 6 years later as a result of reduced annual energy consumption by 40 percent through recent renovations. The entire complex is currently managed by Washington Holdings, a real estate firm also known as Union Square LLC which is based in Seattle.

One Union Square is an aluminum clad 456-foot (139 m) skyscraper consisting of 36 floors with 2 floors below ground. Construction of this class A office building was completed 43 years ago in 1981. It is the first office building in Seattle to house all life-support systems in one location. The architect of One Union Square was TRA.

Construction on Two Union Square began in 1987 and was complete by 1989. When accounting for the tip of the flag pole, the 797-foot (243 m) building is the third-tallest building in the Seattle skyline. The Seattle-based architectural firm NBBJ designed the tower, which was dedicated on July 29, 1989. Two Union Square has 56 floors with 1,126,428 square feet (104,649 m) of rentable space, and an underground concourse connecting to the Seattle Hilton Hotel, and shopping at Rainier Square. It is the first skyscraper to use 19,000 lbs/in. high-strength concrete.

• List of tallest buildings in the United States

• Aïtcin, Pierre-Claude (1998). High-performance Concrete (1st ed.). London: E & FN SPON. ISBN 0-419-19270-0.

• Union Square official website
• Seattle Virtual tour at Bohonus VR Photography