Marceline Day (April 24, 1908 - February 16, 2000) was an American motion picture actress whose career began as a child in the 1910s and ended in the 1930s.

Born Marceline Newlin in Colorado Springs, Colorado and raised in Salt Lake City, Utah, she was the younger sister of film actress Alice Day.

Marceline Day began her film career after her sister, Alice Day, became a featured actress as a Mack Sennett "Bathing Beauty" in one and two-reel comedies for Keystone Studios. Day made her first film appearance alongside her sister in the 1924 Sennett comedy Picking Peaches before being cast in a string of comedy shorts opposite actor Harry Langdon and a stint in early Hollywood Westerns opposite such silent film cowboy stars as Hoot Gibson, Art Acord and Jack Hoxie. Gradually, Day began appearing in more dramatic roles opposite such esteemed actors of the era as Lionel Barrymore, John Barrymore, Norman Kerry, Ramón Novarro, Buster Keaton, and Lon Chaney.

In 1926, Marceline Day was named one of the thirteen WAMPAS Baby Stars, a promotional campaign sponsored by the Western Association of Motion Picture Advertisers in the United States, which honored thirteen young women each year who they believed to be on the threshold of movie stardom. Other notable recipients that year were Joan Crawford, Mary Astor, Janet Gaynor and Dolores del Río. The publicity from the campaign added to Day's popularity and in 1927 she appeared opposite John Barrymore in the romantic adventure The Beloved Rogue.

Marceline Day is probably best recalled for her appearances in the now lost 1927 Tod Browning directed horror classic London After Midnight opposite Lon Chaney and Conrad Nagel, her role as Sally Richards in the 1928 comedy The Cameraman opposite Buster Keaton, and the 1929 drama The Jazz Age opposite Douglas Fairbanks. By the late 1920s, Day's career had eclipsed her sister Alice's, who herself was quite a publicly popular actress. The two would appear together onscreen again in the 1929 musical The Show of Shows.

Although Day transitioned into talkies with little problem, her film roles gradually became lesser in quality and she began working primarily for lower-rung film studios. By 1933, Marceline Day made the transition back to the Western genre, appearing in "B" Westerns starring Tim McCoy, Hoot Gibson, Ken Maynard, Jack Hoxie, and John Wayne. Her last film was The Fighting Parson, with Gibson. After her retirement, Day rarely spoke of her years as an actress and never spoke to reporters or granted interviews.

Marceline Day first married silent film producer Arthur J. Klein, then, after divorcing, married John Arthur. She had no children with either husband. She died of natural causes in Cathedral City, California, USA at the age of 91 and was cremated. Her ashes were given to family.

The younger sister of actress Alice Day, Marceline Day achieved stardom in the mid-'20s, appearing opposite such stars as John Barrymore and Lon Chaney. Adept at comedy, she also starred with such top comics as Buster Keaton and Harry Langdon. Her career faltered in the early '30s, however, and she was soon reduced to appearing in low-budget thrillers and action pictures. She retired in the mid-'30s.