Zookeepers in Adelaide are farewelling one of their biggest and most popular attractions — the much-loved hippopotamus Brutus, who was euthanased this morning at the age of 54.

The hippo was one of the largest and longest-standing residents at Adelaide Zoo, which said he was the oldest hippo in Australia and the second oldest hippo in captivity anywhere in the world.

He was euthanased by zoo staff this morning, after his health condition deteriorated.

Adelaide Zoo said he lived well beyond the average lifespan of a hippo, and nothing more could be done to improve his quality of life.

"Brutus was unable to hold off the advance of time and our latest aged-animal assessment revealed that there was nothing else that could have been done to improve the quality of his life," Zoos SA chief executive Elaine Bensted said.

Adelaide Zoo is currently closed because of coronavirus.

"We know people from all over South Australia would ordinarily come into the zoo to pay their respects to Brutus, however in the current climate of social distancing, we've had to think of new ways for the public to farewell and celebrate our animals from afar," Ms Bensted said.

"That's why we're asking the public to celebrate Brutus's life at home by enjoying a slice or two of watermelon, one of his favourite foods.

Brutus was born at Sydney's Taronga Zoo in 1965, and moved to Adelaide Zoo in 1975.

His female partner Susie died in 2017 at the age of 49 and, at the time, was the oldest female hippopotamus in captivity.

Adelaide Zoo also has a pygmy hippopotamus called Obi.

Ms Bensted said it was unlikely the zoo would replace Brutus with another Nile hippopotamus.

She said the zoo would now focus on breeding pygmy hippos instead.

"We're hopeful we'll be able to get a partner for Obi and look at breeding pygmy hippos, but Adelaide Zoo's probably not large enough to look at bringing younger hippos in," she said.