Mkhulise was on loan last season to Black Leopards where he scored three times in 29 league appearances for the club.
Accompanying him on loan from Sundowns to Lidoda Duvha was Khayalihle Shozi - another player who has just recently been signed by Richards Bay.
The duo were key performers under Leopards coach Jean Francois Losciuto as the club reached the PSL Play-offs, and the Giyani-based outfit had been hopeful that Sundowns would loan them back again.
"We have signed Khaya Shozi and we have signed Siphelele Mkhulise - both from Mamelodi Sundowns," Richards Bay FC chairman Sifiso Biyela tells KickOff.com.
"We have signed Njabulo Ngcobo, Zola Jingxi and Simphiwe Lumkwana from AmaZulu, also on loan.
"We have also signed Stanley Muishond from Stellenbosch FC and former Thanda Royal Zulu goalkeeper Xolani Ngcobo."
As previously revealed by this website, the KZN Rich Boys have also signed young Pirates defender Sandile Mthethwa although he is currently not with the team as he's part of Stuart Baxter's CHAN squad that will face Botswana this coming weekend.
Former Thanda Royal Zulu coach Roger Sikhakhane, who helped AmaBhubesi win the NFD title last season, is in charge of the team.
Sikhakhane's new kids on the block will be part of this weekend's KZN Premier's Cup, a two-day tournament that also features Uthongathi FC, Golden Arrows, AmaZulu, Royal Eagles, Real Kings and Maritzburg United.