The truck was transporting a cobalt-60 teletherapy source from a hospital in the northern city of Tijuana to a radioactive waste storage centre when it was stolen in Tepojaco near Mexico City, the International Atomic Energy Agency said.
"At the time the truck was stolen, the source was properly shielded. However, the source could be extremely dangerous to a person if removed from the shielding, or if it was damaged," it said.
The IAEA said it was informed about the December 2 theft by Mexico's CNSNS nuclear security authority.
The material could not be used in a conventional nuclear weapon but could in theory be put in a so-called "dirty bomb" -- an explosive device spreading the radioactive material over a wide area.
Experts have long warned about the danger posed by the large amounts of such material held in hospitals and other locations around the world under insufficient security.