Melissa Chessington Leo (born September 14, 1960) is an American actress. She is the recipient of several accolades, including an Academy Award, a Primetime Emmy Award, a Golden Globe Award, a Screen Actors Guild Award, and two Critics' Choice Awards.

After appearing on several television shows and films in the 1980s, Leo became a regular on the television shows All My Children, for which she was nominated for a Daytime Emmy Award, and The Young Riders. Her breakthrough role came in 1993 as detective and later sergeant Kay Howard on the television series Homicide: Life on the Street (1993–1997).

Leo received critical acclaim for her performance as Ray Eddy in the 2008 film Frozen River, earning her several nominations and awards, including an Academy Award nomination for Best Actress. In 2010, Leo won several awards for her performance as Alice Eklund-Ward in the film The Fighter, including the Academy Award for Best Supporting Actress.

In 2013, she won a Primetime Emmy Award for her guest role on the television series Louie. She starred in the 2015 Fox event series Wayward Pines as Nurse Pam. She then starred in the 2017 Netflix film The Most Hated Woman in America as American Atheists founder Madalyn Murray O'Hair.

Leo was born in Manhattan and grew up on the Lower East Side. She is the daughter of Margaret (née Chessington), a California-born teacher, and Arnold Leo III, an editor at Grove Press, fisherman, and former spokesman for the East Hampton Baymen's Association. She has an older brother, Erik Leo. Her paternal aunt is art historian Christine Leo Roussel. Leo's parents divorced, and her mother moved them to Red Clover Commune, in Putney, Vermont.

Leo began performing as a child with the Bread and Puppet Theater Company. She attended Bellows Falls High School in Bellows Falls, Vermont, and studied acting at Mountview Academy of Theatre Arts in London and SUNY Purchase, but did not graduate, choosing to leave school and move to New York City to begin auditioning for acting jobs. Leo spent summers at her father's house in Springs, a section of East Hampton, New York.

Leo's acting debut came in 1984, for which she was nominated for a Daytime Emmy at the 12th Daytime Emmy Awards for Outstanding Ingenue/Woman in a Drama Series for All My Children. Following this, Leo appeared in several films, including Streetwalkin', A Time of Destiny, Last Summer in the Hamptons, and Venice/Venice. She also had several appearances on television, most notably her role as Det. Sgt. Kay Howard on Homicide: Life on the Street until 1997. Three years later she reprised her role in the television film Homicide: The Movie. After a brief hiatus from acting, Leo's breakthrough came three years later in the Alejandro González Iñárritu film, 21 Grams released to critical acclaim. Leo appeared in a supporting role alongside Sean Penn, Naomi Watts, Benicio del Toro, and Clea DuVall. Leo shared a Best Ensemble Acting award from the Phoenix Film Critics Society in 2003 and the runner-up for the Los Angeles Film Critics Association for Best Supporting Actress.

Leo appeared in supporting roles throughout the 2000s including the film Hide and Seek, the independent film American Gun, both in 2005, and a minor role in the comedy Mr. Woodcock. In 2006, she won the Bronze Wrangler at the Western Heritage Awards for Outstanding Theatrical Motion Picture for The Three Burials of Melquiades Estrada shared with Tommy Lee Jones who also produced the film. In 2008, she won the Maverick Actor Award and also the Best Actress award at the Method Fest for Lullaby (2008).