Adeyemi, 23, has moved to Elland Road for the 2015-16 campaign with a view to a permanent transfer next summer.
The 6’1” youngster played previously under Leeds head coach Uwe Rosler at Brentford and will strengthen a pool of midfielders weakened recently by the loss of Luke Murphy to injury.
Adeyemi started his career at Norwich City and spent time on loan at Bradford City, Brentford and Oldham Athletic before joining Birmingham City in 2013.
Cardiff signed him a year ago for a fee reported to be in excess of £1million but his opportunities were limited following the dismissal of former City boss Ole Gunnar Solskjaer in September.
Leeds have been working to negotiate an option to take Adeyemi on a full-time basis at the end of the forthcoming term and a statement from the club said he had come to Yorkshire “with a view to a permanent deal.”
His arrival follows that of goalkeeper Charlie Horton, defender Sol Bamba and strikers Chris Wood and Lee Erwin.
United are now prioritising a left winger and are understood to have taken an interest in Brazilian Raphael Martinho.
Martinho was monitored by Leeds owner Massimo Cellino last season but complications with his passport are believed to have prevented a deal.
The 27-year-old currently plays for Verona in Serie A as part of a co-ownership deal with Catania.