American Conservatory Theater (A.C.T.) is a large non-profit theater company in San Francisco, California, that offers both classical and contemporary theater productions. A.C.T. was founded in 1965 in Pittsburgh, Pennsylvania in conjunction with the Pittsburgh Playhouse and Carnegie Tech by theatre and opera director William Ball. By invitation from San Francisco philanthropists and officials, Ball relocated the company to San Francisco and astonished the theatre world by presenting twenty-seven fully staged productions in rotating repertory, in two different theatres, during the first 40-week season. San Francisco Chronicle critic Paine Knickerbocker acclaimed Ball's opening performance of Molière's Tartuffe as "a screaming, bellowing unbelievable triumph."

A.C.T.'s original twenty-seven member acting company featured the talents of René Auberjonois, Peter Donat, Richard Dysart, Michael Learned, Ruth Kobart, Paul Shenar, Charles Siebert, Ken Ruta, and Kitty Winn among many others. Ball's mid-1970s productions of Shakespeare's Taming of the Shrew, starring Marc Singer, and Rostand's Cyrano de Bergerac, starring Peter Donat and Marsha Mason were televised by PBS and are available on video.

In the mid-1980s Ball, suffering from exhaustion and under accusations of financial mismanagement, was forced to relinquish his post as artistic director. He was succeeded by A.C.T. founding member and stage director Edward Hastings, who revived the company's fortunes until the Geary Theatre was severely damaged by the 1989 Loma Prieta earthquake. The company continued performing in a variety of San Francisco venues while laying the groundwork for its restoration.

Since 1992, Carey Perloff has served as A.C.T.'s Artistic Director. A.C.T. is now financially secure and has achieved continued acclaim for its work. In 2007, A.C.T. released a cast album of Perloff's production of the Bertolt Brecht and Kurt Weill musical Happy End, produced and mixed by Lucas Arts studios. It includes the full score and is the first English language recording of this musical.

A.C.T.'s founder's vision was for it to be both a theater company and acting school. The conservatory currently offers a wide range of classes and is accredited to grant Master of Fine Arts degrees for actors. Its MFA program is extremely competitive, admitting only eight students per year among hundreds who audition. It was ranked by US News & World Report as one of the top five actor training programs in the U.S., along with schools like Juilliard, Yale, and NYU. The current director of the conservatory is Melissa Smith. Among the many notable alumni of the MFA program are Denzel Washington, Annette Bening, Benjamin Bratt, Carlos Bernard, Amy Irving, Wynn Harmon, Elizabeth Banks, Anna Belknap, Dileep Rao, Marsha Mason, Danny Glover, Isiah Whitlock, Jr., Tom O'Brien, Anna Deavere Smith, Omar Metwally, Steven W. Bailey, and Anika Noni Rose. In addition to the MFA program, ACT offers training through the Studio A.C.T., the Summer Training Congress, and Young Conservatory programs. Alumni of these programs include Nicolas Cage, Teri Hatcher, Milo Ventimiglia, Winona Ryder, Camryn Manheim, Darren Criss, and Chris Pine.

A.C.T.'s home in San Francisco is the eponymous American Conservatory Theater, previously known as the Columbia Theater and then the Geary Theater until being renamed in September 2006.

The first person to be given the title sound designer in regional theatre was Dan Dugan at A.C.T. in the late 1960s. The term Sound Design was introduced to the film world when Francis Ford Coppola directed (and for which his father, Carmine Coppola, arranged the music and Charlie Richmond was the sound designer) a production of Private Lives at A.C.T., while the final cut of the film The Godfather was being edited in 1972.