Washington County is a county in Texas. As of the 2020 census, the population was 35,805. Its county seat is Brenham, which is located along U.S. Highway 290, 72 miles northwest of Houston.  The county was created in 1835 as a municipality of Mexico and organized as a county in 1837.  It is named for George Washington, the first president of the United States.

Washington County comprises the Brenham, TX Micropolitan Statistical Area, which is also included in the Houston-The Woodlands, TX Combined Statistical Area.

Washington-on-the-Brazos in the county is notable as the site of the signing of the Texas Declaration of Independence during the Convention of 1836. Reflecting the county's history as a destination of mid-19th-century German immigrants who came after the 1848 German revolutions, in the 2000 US Census, more than one third of residents identified as being of German ancestry.