The Blues have stumped up the Spain international's buy-out clause as Jose Mourinho revamps his forward line, but how much of an improvement is the 25-year-old striker?
Jose Mourinho will be hoping the Brazil-born attacker can finally provide the firepower his side have desperately lacked, with last season's trio of Samuel Eto'o, Fernando Torres and Demba Ba all struggling.
Cameroon skipper Eto'o has since departed the club after his contract expired, the subject of "old man" jibes from his manager, though Ba and Torres remain.
As a collective, the three-piece managed just 19 Premier League goals last season, while Costa netted 27 in La Liga alone, with his goals driving Atleti towards the title.
Not only did he score more often, but he also missed fewer clear-cut chances. The 25-year-old's big chance conversation rate across the league and Champions League stood at 50 per cent, while it was just 44% for the Chelsea strikers.
Check out how Chelsea's latest signing stacks up against his new Stamford Bridge attacking team-mates...