Official: Diego Costa joins Atletico Madrid
Atletico Madrid and Chelsea agreed on a fee and Costa has signed his contract at the club and he will wear number 18 during his next chapter at the club.Europa League Final 2018: Marseille vs Atlético Madrid live
Update:

Diego Costa will play with Atletico Madrid starting on January 1st. The 28-year-old signed his contract at the club on Wednesday and will move from Chelsea Football Club.
He will join Diego Simeone's training sessions immeditaely and will wear the number 18 during his second stint at the club.
Costa delighted to be back "home"
"I’m very happy to return home, I’ve always said it, Atlético 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," Costa said on Atletico's official website.
He also thanked "the effort that Miguel Ángel Gil has made especially, and the whole club for making this move forward, I’m very grateful to everyone.”