Chelsea have signed Juventus striker Gonzalo Higuain on loan for the remainder of the season with an option to buy him for 36m euros (£31.3m).
The Blues also have the option to extend the Argentine's loan deal until June 2020 for 18m euros (£15.6m).
Higuain, 31, spent the first half of the season on loan at AC Milan, scoring eight goals in 22 games.
"When the opportunity to join Chelsea presented itself I had to take it," said Higuain.
"It's a team I've always liked that has a lot of history, a wonderful stadium and they play in the Premier League, a league I've always wanted to play in."
As the deal was not completed before Wednesday's midday deadline, he cannot play in Thursday's Carabao Cup semi-final second leg against Tottenham.
The striker played under Chelsea boss Maurizio Sarri while at Napoli in 2015-16, equalling the Serie A goalscoring record with 36 goals.
"He is a very strong striker, especially in my first season at Naples," said Sarri. "He did very very well. For sure, he is one of the best strikers in my career.
"I hope that he will start scoring for us. I know he has had some difficulties at the moment."
Higuain added: "I now hope I can give back that trust Chelsea have shown me out on the pitch. I can't wait to start and I hope to adapt as soon as possible."
Chelsea director Marina Granovskaia said Higuain was the club's number one target in the January transfer window and arrives "with a proven record at the highest level".
"He has previously worked with Maurizio to great effect and is familiar with how the coach likes to play," added Granovskaia.
"This was not an easy deal for us to do because of the numerous parties involved, but we are delighted we were able to make it happen."
Higuain was linked with Arsenal when Juventus signed him for a then domestic record £75.3m from Napoli in July 2016.
After scoring 40 league goals in two seasons, he moved to Milan in August, with the option of making the deal permanent at the end of the season for a fee of 36m euros.
His spell at Milan began well, with six goals in five games, but he only found the net once in his final 12 appearances.
Chelsea are currently fourth in the Premier League, three points ahead of Arsenal who beat them 2-0 at Emirates Stadium on Saturday.