The former Kawasaki Frontale player joined the south-coast club in 2021 and spent the 2021/22 season on loan at Belgian side Royale Union Saint-Gilloise.
He made his Brighton debut in August 2022 and has 13 goals and 12 assists in 52 outings for the club.
Mitoma holds the record for the most Premier League goals scored in a season by a Japanese player, finishing the 2022/23 campaign with seven goals, one more than the previous record held by Shinji Okazaki and Shinji Kagawa.
Brighton finished sixth in the Premier League last season, qualifying for the Europa League.
Mitoma has three goals and three assists in eight top-flight appearances during the current campaign.
Brighton are in sixth place ahead of Saturday's match at champions Manchester City, who were one of the clubs reportedly interested in signing him.
"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."
