The 3-year-old lion is part of a rehabilitation program in Nairobi aimed at rescuing orphaned or injured animals.
He was sterilized "for the purpose of controlling breeding in the place of captivity", the Kenya Wildlife Service (KWS) said.
But the decision sparked an outcry.
Some Kenyans would have preferred that the KWS release the feline in the wild, without having had it sterilized beforehand, to increase the population of the endangered lions.
KWS officials had to justify their decision in a press release on Saturday: "When wild animals are bottle-fed, they lose their natural instincts and if released into the wild, they are vulnerable."
Since breeding “is not permitted in captive centers,” officials ordered a vasectomy,” they added.
Nairobi National Park is home to many endangered animal species, which the public can see grazing against the backdrop of skyscrapers.
But big cats are under increasing pressure in one of Africa's fastest growing cities, especially on former hunting grounds.
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