Maj. Marius Els, 40, also a part-time farmer with land along the Vaal River, was dragged into the water and killed Saturday afternoon by his hippo Humphrey, the Afrikaans language daily newspaper Beeld reported.
The hippo was shot and killed, authorities said.
Humphrey had been cared for by previous owners for more than four years after being was rescued as a calf during a flood. Els took possession after the hippo became too large for the previous caretakers.
Neighbors and friends said they warned Els the hippo was a wild animal, not a pet, and should be treated as such.
"We warned him. He had no ears, " a friend said after the attack.
Els had described the hippo as a "loving and gentle giant."
Jaco Pieterse, representative of the SPCA, warned that wild animals belong in the wild and said the organization strongly disapproved of keeping the animals as pets.