The heaviest dog ever recorded by Guinness World Records was Zorba.
Zorba, formally named Aicama Zorba of La-Susa, was an Old English Mastiff owned by Chris Eraclides. Guinness records and historical editions reported several weights for him, including about 330 pounds in 1989; other references give a peak figure around 343–345 pounds.
Zorba was remarkable for his length as well as his weight. He measured roughly 8 feet 3 inches from nose to tail and stood about 37 inches at the shoulder. Guinness also recognized him as the longest dog ever recorded, making him one of the most physically imposing dogs in record-book history.
The title needs historical context: Guinness World Records later stopped accepting individual “heaviest pet” records, partly because weight-based records could encourage unhealthy overfeeding. That policy does not erase Zorba’s earlier recognition; it means the category is no longer routinely updated.