William Gabriel Tachau, AIA, (April 1875 – January 1969), was an American architect active in early- to mid-twentieth-century New York City. With Lewis Pitcher, he was a partner in the architectural firm of Pitcher & Tachau from 1904 to 1919 when he established the firm of Tachau & Vought. Both firms from 1918 onward specialized in mental hygiene hospitals. The firm moved from 109 Lexington Avenue to 102 East 30th Street around 1923 and remained at that address and that name even after Vought left.