The two clubs submitted a deal sheet ahead of the 11pm deadline to extend proceedings, with the transfer finalised shortly afterwards. Brighton will pay an initial £12.5million, with performance-related add-ons capable of pushing the overall package beyond £15million, making Azeez the most expensive departure in Millwall's history.
Sporting director Mike Cave said the club had moved for one of the standout performers outside the top flight last season.
"He arrives off the back of an excellent season with Millwall, having played a key role for them as they finished in the playoffs," Cave told Brighton's official website.
"He was one of the most exciting attacking talents in the Championship, and we're delighted to have finally agreed his transfer, and we now look forward to working with him."
Azeez made 77 appearances for Millwall following his move from Reading in 2024, scoring 15 goals and providing 13 assists, form that saw him named in both the PFA and EFL Championship teams of the season. He has also enjoyed a rapid rise on the international stage, scoring twice on his Nigeria debut in a 2-0 win over Zimbabwe in June before adding an assist in a 3-0 victory over Jamaica in his following appearance.
The winger's route to the top flight has taken him through non-league football at Northwood and Wealdstone before a breakthrough at Reading paved the way for his switch to Millwall. His departure represents a significant financial boost for the Championship club, who can now reinvest the fee before the domestic window closes, while Brighton add further attacking depth to a squad already strengthened by the arrivals of Chema Andrés and Luka Vuskovic this summer.
Millwall had rejected at least three previous bids from Brighton before finally accepting the improved offer on deadline day, having initially hoped to keep hold of a player who had become central to their promotion push.
Manager Fabian Hürzeler had made Azeez a key target throughout the summer window, valuing his direct running and goal threat as qualities that could add a new dimension to Brighton's attack, particularly with the club also competing in the Conference League this season and requiring squad depth across multiple competitions.
Azeez becomes Brighton's 11th signing of a busy summer window, one funded in part by significant outgoing sales including Carlos Baleba's £70million move to Manchester United, underlining the scale of the club's recruitment strategy this year.