San Lorenzo is a town located in Alameda County, California. As of the 2000 census, the town had a total population of 21,898.

As of the census of 2000, there are 21,898 people, 7,500 households, and 5,677 families residing in the town. There are 7,609 housing units at an average density of 1,060.6/km.

There are 7,500 households out of which 34.7% have children under the age of 18 living with them, 58.1% are married couples living together, 12.6% have a female householder with no husband present, and 24.3% are non-families. 19.1% of all households are made up of individuals and 11.5% have someone living alone who is 65 years of age or older. The average household size is 2.92 and the average family size is 3.34.

In the town the population is spread out with 25.2% under the age of 18, 8.0% from 18 to 24, 29.5% from 25 to 44, 21.3% from 45 to 64, and 16.0% who are 65 years of age or older. The median age is 38 years. For every 100 females there are 94.0 males. For every 100 females age 18 and over, there are 91.3 males.

The median income for a household in the town is $56,170, and the median income for a family is $61,787. Males have a median income of $43,626 versus $34,531 for females. The per capita income for the town is $21,922. 5.4% of the population and 3.7% of families are below the poverty line. Out of the total people living in poverty, 6.4% are under the age of 18 and 4.2% are 65 or older.