First Monday in October is a 1981 American comedy drama film from Paramount Pictures, produced by Paul M. Heller and Martha Scott, directed by Ronald Neame, that is based on the 1978 play by Jerome Lawrence and Robert Edwin Lee. The film stars Walter Matthau (for which he was nominated for a Golden Globe Award for Best Actor – Motion Picture Musical or Comedy) and Jill Clayburgh (for which she was nominated for a Golden Globe Award for Best Actress – Motion Picture Musical or Comedy). The cast also co-stars Jan Sterling in her final feature film role.

Paramount Pictures originally scheduled First Monday in October for release in February 1982, but President Ronald Reagan's nomination of Sandra Day O'Connor as the first female Supreme Court justice on July 7, 1981, forced the film's release a month after her nomination, in August 1981.

The film's title refers to the day that the Supreme Court commences its annual term, which continues until June or early July of the following year.