Chelsea have completed the signing of Egypt winger Mohamed Salah from FC Basel.
Manager Jose Mourinho made the announcement prior to his side's FA Cup fourth-round tie against Stoke City at Stamford Bridge on Sunday.
The fee involved for Salah has not been revealed yet but Sky sources understand it is in the region of £15million.
The 21-year-old has penned a five-and-a-half-year deal with the club and will wear the number 15 shirt.
Speaking to the club's official website, Salah said: "I'm very happy to sign for such a big club. I hope I can make the Chelsea supporters happy."
The winger becomes the club's third signing of the winter transfer window following Chelsea's capture of Bertrand Traore and Nemanja Matic.
Salah, who becomes the first Egyptian to sign for Chelsea, will give Mourinho further options in the attacking third of the field following the recent departures of Juan Mata and Kevin De Bruyne.
Speaking on Friday, Mourinho revealed his admiration for the attacking midfielder who has scored 17 goals in 27 appearances for Egypt.
"We like the fact that he can play the same way our attacking players do, which is players with adaptability, to play right, left, or behind the striker," Mourinho said.
"He's young, he's fast, he's creative, he's enthusiastic. When we analysed him he looks the kind of humble personality on the pitch, ready to work for the team and to work and to adapt himself to a new life.
"We think with him and (Andre) Schurrle, (Eden) Hazard, Oscar and Willian we will be fine.
"We lose an experienced top player like Juan, but we bring in a young player with great potential.
"He has some experience in Europe, the Europa League, Champions League, so he's not a naive boy arriving in the jungle.
"Hopefully he comes and hopefully with the talented players we have around in the same positions we all can develop together."
Salah, who has scored three times in four matches against Chelsea in the last 12 months, was said to be the primary January target for Liverpool manager Brendan Rodgers.