Cuarto milenio is a Spanish television program directed and presented by journalist Iker Jiménez and Carmen Porter. The program has been broadcast weekly on television channel Cuatro since November 2005, being the channel's longest running program.

The program explores a wide variety of topics, usually related to mystery and enigmas, though frequently reaching afield. They include conspiracy, archaeology, history, criminology, astronomy, medicine, physics, zoology, psychology, parapsychology and ufology. It regularly feature experts in various disciplines, like psychiatrists, physicists, physicians, historians, anthropologists, criminologists, astronomers, naturalists, writers or journalists.Cuarto Milenio has been acknowledged to have an established place on Spanish television, not only due to its longevity, but also to its following and recognizability, having been considered a television phenomenon. Alternatively, it has been criticized as pseudoscientific, pointing to its treatment of esoteric and conspiratorial fields, and praised as a program of investigation and divulgation, standing out by its consistent presence of scientific authorities and specialists as guests.The program has spawned two spin-offs; La mesa del coronel, a geopolitical program presented in 2019 by Col. Pedro Baños, and Horizonte, produced by the same team and ongoing since 2020.