The Union Trust Building is a commercial building in Seattle, Washington, United States. Located in the city's Pioneer Square neighborhood, on the corner of Main Street and Occidental Way South, it was one of the first rehabilitated buildings in the neighborhood, which is now officially a historic district. In the 1960s, when Pioneer Square was better known as "Skid Road", architect Ralph Anderson purchased the building from investor Sam Israel for $50,000 and set about remodeling it, a project that set a pattern for the next several decades of development in the neighborhood. Anderson also rehabilitated the adjacent Union Trust Annex.

William Boone was an American architect who practiced mainly in Seattle, Washington from 1882 until 1905. He was one of the founders of the Washington State chapter of the American Institute of Architects as well as its first president. For the majority of the 1880s, he practiced with George Meeker as Boone and Meeker, Seattle's leading architectural firm at the time. In his later years he briefly worked with William H. Willcox as Boone and Willcox and later with James Corner as Boone and Corner. Boone was one of Seattle's most prominent pre-fire architects whose career lasted into the early 20th century outlasting many of his peers. Few of his buildings remain standing today, as many were destroyed in the Great Seattle fire including one of his most well known commissions, the Yesler – Leary Building, built for pioneer Henry Yesler whose mansion Boone also designed. After the fire, he founded the Washington State chapter of the American Institute of Architects and designed the first steel frame office building in Seattle, among several other large brick and public buildings that are still standing in the Pioneer Square district.