The Castro District, commonly referred to as the Castro, is a neighborhood in Eureka Valley in San Francisco. The Castro was one of the first gay neighborhoods in the United States. Having transformed from a working-class neighborhood through the 1960s and 1970s into one that came to represent some of the highest geographical and communal concentrations of same-sex coupling, the Castro remains one of the most prominent symbols of lesbian, gay and bisexual activism and events in the world.

San Francisco's gay village is mostly concentrated in the business district that is located on Castro Street from Market Street to 19th Street. It extends down Market Street toward Church Street and on both sides of the Castro neighborhood from Church Street to Eureka Street. Although the greater gay community was, and is, concentrated in the Castro, many gay people live in the surrounding residential areas bordered by Corona Heights, the Mission District, Noe Valley, Twin Peaks, and Haight-Ashbury neighborhoods. Some consider it to include Duboce Triangle and Dolores Heights, which both have a strong LGBTQ presence.

Castro Street, which originates a few blocks north of the Castro Street Business District at the intersection of Divisadero and Waller Streets, runs south through Noe Valley, crossing the 24th Street business district and ending as a continuous street a few blocks farther south as it moves toward the Glen Park neighborhood. It reappears in several discontinuous sections before ultimately terminating at Chenery Street, in the heart of Glen Park.

Castro Street was named after José Castro (1808–1860), a Californian leader of Mexican opposition to U.S. rule in California in the 19th century, and alcalde of Alta California from 1835 to 1836. The neighborhood known as the Castro, in the district of Eureka Valley, was created in 1887 when the Market Street Railway Company built a line linking Eureka Valley to downtown.

In 1891, Alfred E. Clarke built his mansion at the corner of Douglass and Caselli Avenue at 250 Douglass which is commonly referenced as the Caselli Mansion. It survived the 1906 earthquake and fire which destroyed a large portion of San Francisco.

During the California Gold Rush and in its aftermath, a substantial Finnish population settled in San Francisco. Finnish Club No. 1 was established in the Castro District of San Francisco in 1882. Soon after, two "Finnish Halls" were erected nearby. One was located at the corner of 24th Street and Hoffman Street. The other hall was located on Flint Street, on the "Rocky Hill" above Castro, an area densely populated by Finns at the time, consequently nicknamed "Finn Town".

In 1899, the First Finnish Lutheran Church was founded on 50 Belcher Street, in what then was considered part of the Eureka Valley district of San Francisco, but what is located on the outskirts of what today is best known as the Castro District. Next to it, on September 17, 1905, the cornerstone was laid for the Danish St. Ansgar Church at 152 Church Street, between Market Street and Duboce Avenue. In 1964, St. Ansgar merged with First Finnish Lutheran Church. The name for the united church, St. Francis Lutheran Church, was derived from San Francisco.

Before the 1906 San Francisco earthquake, nearly all the children attending the McKinley School (now McKinley Elementary School) at 1025 14th Street (at Castro) were Finnish. Following the earthquake, a large number of Finns from San Francisco and elsewhere moved to Berkeley, where a Finnish community had been established already before the earthquake. The brick and wood frame of the St. Francis Lutheran Church building survived the 1906 San Francisco earthquake and then was used for several months as an infirmary. Following the earthquake, the same year, Finns founded the Lutheran Church of the Cross in Berkeley, at University Avenue, where the Lutheran congregation still operates today.