The animal, which weighs over a ton, reportedly bit Marius Els, dragged him into the water and killed him on his 400-acre farm in South Africa.
The man's mutilated body was found submerged in the river where years earlier the creature had been rescued from a flood.
The 40-year-old farmer, who said the animal was like a "son" to him, was confirmed dead by medics early on Sunday morning.
Netcare 911 spokesman Jeffrey Wicks reported paramedics attended Els' home on the banks of the Vaal River in Free State province.
He said: "Paramedics responded to the scene to find that the man had been bitten several times by the animal.
Humphrey, who is six, was found as a calf and had been raised as a pet in Free State.
Hippos are very aggressive towards humans, whom they commonly attack whether in boats or on land with no apparent provocation. »