AC Milan have signed Antonio Donnarumma, the brother of Gianluigi, on a four-year contract.
Antonio Donnarumma, a 27-year-old who is also a goalkeeper, has moved on a permanent transfer from Asteras Tripolis.
His deal was confirmed on Wednesday, a day after Gianluigi signed a new contract at San Siro, also until 2021.
The elder Donnarumma is a former Milan youth keeper, who left to move to Genoa in 2012, where he only made one Serie A appearance across four years.
Antonio has also spent loan spells in Serie B with Piacenza, Gubbio and Bari.
He spent one season with Asteras in Greece's top flight and has now moved back to Milan, where he will provide back-up to Gianluigi along with Gabriel and Marco Storari.
Milan sporting director Massimiliano Mirabelli insists he was not just signed to help keep Gianluigi happy.
"The first thing we had to do was to renew Gigio's contract, but I immediately had the idea of bringing in his brother Antonio too," he said.
"The technical evaluation comes first and I think Antonio can be a 12th man for Milan.
"He is not a postage parcel like someone said. I'm sorry for him if certain things have been written. Vincenzo Montella also endorsed this signing.
"I know him well and I wanted to bring the family back together. I'm proud to present the Donnarumma pair."
Antonio added: "I will be in the dressing room with my brother, which makes me very happy. It is a beautiful thing to be back at Milan."
He is Milan's eighth signing of the transfer window, after Hakan Calhanoglu, Andrea Conti, Franck Kessie, Ricardo Rodriguez, Mateo Musacchio, Andre Silva and Fabio Borini.