When Dinosaurs Ruled the Earth is a 1970 British science fiction film from Hammer Films, written and directed by Val Guest, and starring Victoria Vetri. It was produced by Aida Young. This was the third in Hammer's "Cave Girl" series, preceded by One Million Years B.C. (1966) and Prehistoric Women (1967); it was followed by Creatures the World Forgot (1971).

Val Guest was an English film director and screenwriter. Beginning as a writer of comedy films, he is best known for his work for Hammer, for whom he directed 14 films, and for his science fiction films. He enjoyed a long career in the film industry from the early 1930s until the early 1980s.