Mitoma has been one of Brighton's best players since breaking into the first team last season.
The 26-year-old joined the Seagulls from Japanese side Kawasaki Frontale in the summer of 2021 and spent a season on loan at Belgium side Union Saint-Gilloise.
Mitoma scored seven goals and provided five assists last season as Brighton finished sixth and qualified for the Europa League for the first time in their history.
This term, he already has three goals and three assists in eight league games, and had started attracting interest from some of the big boys.
His solo effort against Wolverhampton Wanderers was voted the Premier League's goal of the month for August.
"It's great news," manager Roberto De Zerbi said in a club statement on Friday. "Kaoru is our top player."
Technical director David Weir added: "Ensuring that a player of Kaoru's quality will remain with us for the long term is fantastic news. Kaoru adapted to the Premier League last season and has quickly become one of our most important players."
“His success also reflects the work of the departments that played a role in his recruitment, helping him gain important experience on loan and ultimately becoming a big part of Roberto’s first-team group.”