Chelsea completed the signing of Leicester City midfielder Kiernan Dewsbury-Hall after agreeing a £30 million fee, the club announced on Tuesday.
The 25-year-old signed a six-year contract until 2030.
Brighton were on the verge of finalising a deal with Leicester for Dewsbury-Hall in a cash-plus-player transfer which involved Poland midfielder Jakub Moder going the other way.
However, those talks collapsed after Dewsbury-Hall made it clear he wanted to move to London when Chelsea entered the running, and he will be reunited with former Leicester boss Enzo Maresca, who began work as Mauricio Pochettino's successor on Monday.
"It is amazing to be sitting here as a Chelsea player," Dewsbury-Hall said on Tuesday. "This is a club people grow up watching and hoping to one day play for, so it is a privilege to be here.
"I'm really excited to get going and can't wait to show everybody what I can do."
Dewsbury-Hall netted 12 goals and contributed 12 assists in 44 league matches last season, earning him the title of Leicester's Player of the Season as they won the Championship.