The striker has formalized the long-awaited move after Monaco sealed a loan deal for Atletico Madrid winger Gelson Martins, freeing up room for Morata.
The Spaniard has joined Chelsea from Atletico’s cross-town rivals Real Madrid in July 2017 for €65m.
However, the Spain international struggled for form for the majority of his time in the England and has now returned to Spanish football where he will provide competition for Antoine Griezmann and Diego Costa up front for Diego Simeone’s side.
“I’m very happy and proud to be here. I can’t wait to start training, meet my new teammates and to play,” Morata told Atleti’s club website.
“Ever since I was a ball boy for the club, I’ve always dreamt of playing for the team,” Morata has said, in quotes carried by Marca.
“We always had to come to the game two hours earlier and I remember fondly waiting at the entrance with my father, they are such cherished memories.
“I am very proud of this move – I know the club, the stadium, the shirts, the players…it is a really happy moment for me, I cannot wait to get started.
“I have been thinking about this move for a long time and I have been watching a lot of the recent matches too.
“I already have felt like I have been part of this club and now I am extra motivated to give my everything.”
Meanwhile, a statement from Los Rojiblancos read:
“Morata has proven himself as a prolific goalscorer in some of Europe’s most competitive leagues.
“We add a very talented striker to our club, who has the ability to score with his feet and his head thanks to his height (189 cm) and to link-up with our teammates.”