The teen's move was in the works for several weeks and has now been formalised.
Fati joins Real Madrid for a fee of €120,000.
Leganes announced today: "CD Leganés and Real Madrid CF have agreed to the transfer of 18-year-old Lamini Fati. The youth player from the white and blue team will leave the pepinera discipline to continue his career with the Merengues team.
"Lamini joined Leganés in the youth category, passing through all the youth teams. In the 2023/24 season he was part of the Juvenil 'A' team that finished the División de Honor league in third position and reached the quarter-finals of the youth Copa del Rey.
"This season, with the Pepinero reserve team, he has played more than 900 minutes, scoring a goal in the 13th matchday of the Tercera Federation.
"The club wishes him the best of luck in this new stage."