The move was formalised on Monday after the France playmaker completed a medical, though the transfer had been agreed between the two clubs during the World Cup, which Lemar and France went onto win.
“I’m very happy to be here,” he said.
“Antoine and Lucas have told me lots about the club. I’m very happy with the decision I made.
“I know this team demands hard work and I’ll give it my all for the team. If I need to run more, I’ll do it without a problem.”
Amid rumours that he is set to leave Chelsea this summer, Brazilian winger Willian says he is “very happy” at the club and intends to stay.
However, in an interview with Brazilian TV on Sunday, Willian said, “I’m very happy at Chelsea.
“I’m very happy living in London.”
Willian had fallen down the pecking order last season under former manager Antonio Conte, which had fueled speculation of a move away. However, that is no longer an issue, with the Italian having since left the post and been replaced by compatriot Maurizio Sarri.
The Brazilian is due back in London soon to link up with the Chelsea squad after his exertions at the World Cup in Russia this summer.
Chelsea have informed Real Madrid that they will not resume talks about goalkeeper Thibaut Courtois until they secure a replacement, according to Sky Sports.
Ex-Atletico Madrid loanee Courtois, 26, has been linked with a €35 million (Dh150.7m) return to the Spanish capital all pre-season. The Belgium No1 desires being closer to his children and has declined to renew a contract with the Blues that expires next summer.
But he will begin 2018/19 at Stamford Bridge if a successor is not purchased. Everton’s Jordan Pickford and Stoke City’s Jack Butland have been linked to the role.
Courtois has played 154 times for Chelsea since his arrival from Genk in 2011.