Atletico Madrid have confirmed that the deal for Diego Costa to return to the club from Chelsea is now complete.
The two clubs revealed an agreement had been reached last week, with Costa flying to Madrid to undergo a medical and agree personal terms.
Costa left Atletico for Chelsea in 2015 and has won two Premier League titles, but is now back in Madrid.
"The Spain international forward has signed his new contract after successfully passing the medical tests that he underwent in two phases: last Saturday and Monday," Atletico said.
"Starting tomorrow, he will join the team in training, but he will not be registered until the winter market opens on January 1st."
The reason Costa cannot play for Atletico until January is because that's when they will end their transfer ban.
Costa said: "I’m very happy to return home, I’ve always said it, Atletico is my home, I’m very, very happy
"I’m looking forward to start contributing. I’m going to give my all for the team, as I always try to do.
"I am grateful for the effort that Miguel Angel Gil has made especially, and the whole club for making this move forward, I’m very grateful to everyone."
Costa has been given the No.18 shirt.