The 18-year-old defender joins from Botola Pro side Union Touarga in a deal reported by Africa Foot to be worth an initial €700,000, rising to around €800,000 with add-ons, with the Moroccan club retaining a 25 per cent sell-on clause.
Crystal Palace saw off interest from Raja Casablanca to complete the signing, which addresses a need for defensive depth after the club lost both Marc Guéhi to Manchester City and Maxence Lacroix to Chelsea earlier this year.
Majni progressed through Touarga's youth ranks before making his senior debut in June 2025, featuring in the club's Excellence Cup semi-final win over Wydad Fès. He went on to make 17 senior appearances in total, 15 of them last season including 13 starts, operating in a versatile role capable of playing at centre-back or right-back.
His club form was overshadowed by a landmark international achievement. Majni was part of the Morocco squad that won the FIFA Under-20 World Cup in Chile last October, coming off the bench for the final five minutes of a 2-0 victory over Argentina in the final under coach Mohamed Ouahbi.
Aged 17 at the time, he featured in a tournament in which Morocco also defeated Brazil, Spain and France on their way to a first title in the competition. He has since progressed to Morocco's Under-23s, winning two caps in March including a start in a 2-2 draw with Ivory Coast, having also represented the country twice at Under-17 and Under-18 level.
"Palace are one of the biggest clubs in the Premier League and around the world. They have achieved a lot in recent years, so I'm very excited to begin this new adventure with the club," Majni said.
Academy director Gary Issott added: "Taha enjoyed an excellent 25/26 season, gaining valuable senior experience in the Moroccan top-flight and helping Morocco lift the Under-20 World Cup at just 17.
"He is a talented and versatile defender who has impressed us with his performances, and we believe Palace is the right environment for him to continue his development."
Majni will join Darren Powell's Under-21 squad rather than compete immediately for a first-team place, playing across Premier League 2, the Premier League International Cup and the EFL Trophy as he continues his development in English football. Describing himself as a technical defender who enjoys the physical side of the game, he becomes the latest Touarga academy product to move to Europe this summer, following forward Tawfik Bentayeb to Anderlecht and midfielder Adam Baallal to Saint-Étienne.