The midfielder joined Cardiff in a £1.1million deal from Birmingham last summer, but struggled to make an impact in South Wales, with his last appearance for the club coming in March.
The box-to-box midfielder, 23, started his career at Norwich but played under new Leeds boss Uwe Rosler at Brentford as the club suffered play-off final heartbreak against Yeovil in 2013.
Adeyemi travelled to Leeds on Monday for talks and a medical and it is thought a move to the Whites, initially on a season-long loan, will be confirmed on Tuesday.
Rosler has been in the market for a new central midfielder after the club lost Luke Murphy for the opening month of the season to a knee injury. Jamaican midfielder Rudy Austin was released on a free at the end of last season.