The 19-year-old defender made his first-team debut for Chelsea in their Premier League win at Arsenal last month and has featured as a substitute in both rounds of the FA Cup.
He has now completed a switch to the south coast, with Brighton confirming the signing of the England youth international for an undisclosed fee.
Lamptey, also capable of playing in midfield, is now looking to prove himself in the top flight.
"It was a really tough decision to leave as I've been at Chelsea since the age of seven and seen it all the way to the under-23s, and then the first team," he said.
"It was a proud moment when I made my Premier League debut, but I felt that it was the right time to start a new chapter and come to this brilliant club.
"The manager and the club have shown great faith in bringing me here. I just hope I can repay their trust, and help the club reach their goals and objectives.
"They are a team on the up with a great fanbase, training ground and stadium and they have everything I need to become a better player."
Brighton is 15th in the league and travels to fellow struggler West Ham United on Sunday (AEDT).
The Seagulls have also recalled striker Jurgen Locadia from a loan spell with Bundesliga side Hoffenheim, with the Dutchman having scored four goals in 11 league appearances this term.