The Colombian striker has struggled for regular game-time this season and has often found himself on the bench.
However, the Soweto giants have now confirmed that they have reached an agreement with Sundowns for the services of Castro, who will join the club next month.
A statement on Sundowns' website reads:"Mamelodi Sundowns and Kaizer Chiefs have reached an agreement on the transfer for Leonardo Castro to Naturena.
"The Colombian striker will join Kaizer Chiefs in January.
"Mamelodi Sundowns would like to take the opportunity to thank Castro for his contribution and wish him well at his new home."
Since joining Sundowns in the 2015-2016 season, Castro has gone on to make 62 appearances for the club, scoring 12 goals in the process.