The zoo, in Bremerhaven, northern Germany, says the adult males - Z and Vielpunkt - were given an egg which was rejected by its biological parents.
It says the couple are now happily rearing the chick, said to have reached four weeks old.
The zoo made headlines in 2005 over plans to "test" the sexual orientation of penguins with homosexual traits.
Three pairs of male penguins had been seen attempting to mate with each other and trying to hatch offspring from stones.

Since the chick arrived, they have been behaving just as you would expect a heterosexual couple to do
Bremerhaven zoo
The zoo flew in four females in a bid to get the endangered birds to reproduce - but quickly abandoned the scheme after causing outrage among gay rights activists, who accused it of interfering in the animals' behaviour.
The six "gay" penguins remain at the zoo, among them Z and Vielpunkt who are now rearing the chick together after being given the rejected egg.
"Z and Vielpunkt, both males, gladly accepted their 'Easter gift' and got straight down to raising it," said a zoo statement.
"Since the chick arrived, they have been behaving just as you would expect a heterosexual couple to do. The two happy fathers spend their days attentively protecting, caring for and feeding their adopted offspring."
Humboldt penguins are normally found in coastal Peru and Chile, but their numbers have been dwindling due to overfishing, reports the AFP news agency.
'Drive to mate'
There have been previous reports of exclusive male-to-male pairings among penguins, some of which have also included the rearing of chicks.
Homosexual behaviour is well documented in many different animals, but it is not understood in detail, says Professor Stuart West, an evolutionary biologist at the University of Oxford.
Professor West says it has been suggested that homosexual activity could serve various purposes - for instance, it may relate to social bonding and establishment of dominance among bonobo chimps, while in some bird species, females may come together to rear young.
Other animals may simply exhibit a "drive to mate", while others may, like humans, enjoy non-procreative sexual activity.
"Homosexuality is nothing unusual among animals," Bremerhaven zoo said on Wednesday.
"Sex and coupling up in our world do not necessarily have anything to do with reproduction."

source: bbc