Alto (Spanish for "Tall") is a census-designated place[3] adjacent to Mill Valley in Marin County, California.

According to the United States Census Bureau, the CDP covered an area of 0.326 square kilometers (0.126 sq mi), all of it land.

The population density was 5,654.1 inhabitants per square mile (2,183.1/km2).

The racial makeup of Alto was 619 (87.1%) White, 8 (1.1%) African American, 2 (0.3%) Native American, 30 (4.2%) Asian, 1 (0.1%) Pacific Islander, 16 (2.3%) from other races, and 35 (4.9%) from two or more races.

[20] The census reported that 100% of the population lived in households.