Cassius was nearly 5.5m (18ft) long, weighed nearly one tonne and was thought to be at least 110 years old - although no one knew for sure.
The huge saltwater crocodile lived in the sanctuary on an island off the coast of Queensland since being caught in Australia's Northern Territory in the 1980s.
Marineland Melanesia Crocodile Habitat said in a post on social media that Cassius was "our beloved mate" and "a cherished member of our family".
Cassius had previously lived in the wild, where he was known for catching and eating cattle, and attacking boat propellers before being captured.
The habitat also thanked "everyone who visited Cassius throughout his life and offered kindness", adding that the site may be operating "in a limited capacity over the next few days".