The 20-year-old Moroccan international has put pen to paper on a five-year contract with the Bundesliga side.
Ben Seghir, who can operate both in central areas and out wide, had been left out of Monaco's squad for their late win over Strasbourg at the weekend. He made 42 appearances for Les Monegasques across all competitions since making his senior debut in 2022, scoring seven goals and emerging as one of Ligue 1's brightest young talents.
Bayer Leverkusen sporting director Simon Rolfes said the club were delighted to secure the youngster's services. "Eliesse can play wide in attack and also in central areas. He's tough to predict and a top addition for our squad. We can look forward to his displays for Bayer 04," Rolfes has said.
The move comes as part of Leverkusen's attacking overhaul following the departure of Amine Adli to Premier League side Bournemouth. Ben Seghir had been identified as a priority signing for manager Erik ten Hag, who wanted extra creativity and versatility in the final third as the club prepare for another Bundesliga campaign and Champions League challenges.
Ben Seghir, who already has senior caps for Morocco, said he was eager to take the next step in his career. "Leverkusen are a great club and the opportunity to play under Erik ten Hag is something I could not turn down," he has said.
Monaco, meanwhile, are expected to look for attacking reinforcements before the transfer window closes, with head coach Adi Hutter stressing the importance of balancing the squad following Ben Seghir's departure.