The head of the Belize Institute of Archaeology says the destruction was detected late last week.
Only a small portion of center of the pyramid mound was left standing.
Dr. Jaime Awe says he was sickened by the destruction of the Nohmul pyramid and temple platform, which date back about 2,300 years.
Photos of the portion that remained showed what appeared to be classic Mayan-arched chamber dangling above one clawed-out section.
The Nohmul complex sits on private land, but Belizean law states any pre-Hispanic ruins is under government protection.