Robert Jay Arthur, Jr. was a mystery writer known for The Mysterious Traveler radio series and his Three Investigators series of novels. He is frequently confused with the film producer Robert Arthur, who was nine days older than mystery writer Robert Arthur, Jr.

Robert Arthur, Jr. wrote a number of mystery books, mostly for children, and he also worked on the anthology television series Alfred Hitchcock Presents.

In the 1950s and 1960s, as an uncredited ghost editor, Arthur anonymously compiled more than a dozen anthologies of mystery, suspense and supernatural stories which were purportedly edited by Alfred Hitchcock; these books were authorized by the real Hitchcock but were entirely edited by Arthur, who typically included at least one of his own stories in most of the anthologies in addition to ghostwriting for each book a foreword allegedly authored by Hitchcock.

Arthur, along with his writing partner David Kogan, was twice honored by the Mystery Writers of America with an Edgar Award for Best Radio Drama: In 1950, for Murder by Experts, and again in 1953, for The Mysterious Traveler. Robert Arthur, Jr. died in Philadelphia in 1969.