Clifford Martin Eddy Jr. (C. M. Eddy Jr.; January 18, 1896 – November 21, 1967) was an American writer known for his horror, mystery and supernatural short stories. He is best remembered for his work in Weird Tales magazine and his friendship with H. P. Lovecraft.

Eddy was born in Providence, Rhode Island, on January 18, 1896. He went to Classical High School in Providence, and as a child was a precocious reader and writer. He continued to be an avid reader and writer, interested in mythology and the occult. According to his wife Muriel,

Cliff was always interested in the idea of parallel planes—where life on another level, either astral or otherwise, would be similar to that on earth—or where life might exist, but in another time or another form. He was also fascinated by the themes of teleportation, vampirism, ghosts and the mystery of unexplained phenomena...he spent hours in the library researching the unusual, the unique, the bizarre.

He began his career writing short stories for a broad range of pulp fiction magazines such as Weird Tales, Munsey's Magazine, and Snappy Stories. His first published tale, "Sign of the Dragon" (Mystery Magazine, 1919), was a detective story. (In October 2012 it was released as a standalone e-book.)

Various tales of mystery, ghosts, and song-writing (he himself wrote songs, including "When We Met by the Blue Lagoon", "In My Wonderful Temple of Love", "Dearest of All", "Underneath the Whispering Pine", "Sunset Hour", and "Hello Mister Sunshine (Goodbye, Mister Rain)"), continued to appear through 1925 in various magazines. They included "A Little Bit of Good Luck" (a story about songwriting), Munsey's Magazine, 1920; "Moonshine" (ghost story), Action Stories, 1922; "The Unshorn Lamb (another story about songwriting) Snappy Stories, 1922. Some stories written at this time were unpublished: "Pilgrimage of Peril", "The Vengeful Vision" and "A Solitary Solution" (all 1924) until collected in Exit Into Eternity (1973).

The Eddys' first contact with H. P. Lovecraft occurred as early as 1918. They first met face to face in August 1923, according to Muriel Eddy being introduced by Eddy's and Lovecraft's mothers, who were both active in the women's suffrage movement.

Lovecraft frequently visited the Eddys' home on Second Street in East Providence, and later called on them at their home in the Fox Point section of Providence. Eddy was a member of Lovecraft's inner circle of friends and authors, and he and Lovecraft edited each other's works. Both authors were also investigators for Harry Houdini and served the magician as ghostwriters. The two collaborated on The Cancer of Superstition, ghostwritten for Houdini, but the latter's death in October 1926 curtailed the project. (Notes and surviving fragments were published in The Dark Brotherhood and Other Pieces.)

Eddy and Lovecraft took scenic walks, including one to the Old Stone Mill in Newport, Rhode Island; August Derleth later incorporated notes taken by Lovecraft on this occasion into The Lurker at the Threshold (1945).