Howell Mountain is an American Viticultural Area (AVA) located in Napa County, California and the first sub-AVA within the internationally-acclaimed Napa Valley. The 14,080 acres (22 sq mi) region was effectively recognized on January 30, 1984 by the Bureau of Alcohol, Tobacco and Firearms (ATF), Treasury after reviewing the petition submitted by members of the grape-growing and wine-producing industries in the area proposing to establish a viticultural area named "Howell Mountain."