It lives on fire corals of the genus Millepora in the Atlantic Ocean from Florida to southern Brazil.

[1] In his original description of the species, Darwin reported that M. stultus occurred in Singapore and the West Indies.

In 1968, Arnold Ross considered the reports of M stultus from the Pacific Ocean to be erroneous,[2] limiting the type locality to the West Indies.

M. stultus has subsequently been reported from Taiwan in the western Pacific,[3] and those records have been reassessed as referring to the related species Megabalanus ajax.

[4] M. stultus is now thought to occur exclusively in the western Atlantic, including the Caribbean Sea, from Florida to Rio de Janeiro, Brazil.