Defender Levi Colwill has signed a new six-year deal with Chelsea, with an option for a further year.
The England Under-21 international helped Brighton qualify for the Europa League last season, making 17 Premier League appearances while on loan at the club.
The 20-year-old posted a goodbye message to Seagulls fans on Instagram on Tuesday.
“It’s always been an easy decision,” said Colwill.
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“‘As I’ve grown up, all I’ve known is Chelsea. I’ve spoken with the manager [Mauricio Pochettino] and he’s given me reassurance, that trust I needed.
“He’s spoken with me not just as a player but a person. It’s made a big difference.”