The 26-year-old had been training with the Brazilians following his departure from Ea Lla Koto at the end of last season.
He becomes the second Mamelodi-born player to join Sundowns from Free State Stars following the arrival of Lucky Mohomi at the beginning of 2016/17 season.
Pitso Mosimane confirmed on Sunday that they are looking to bring as many Mamelodi-born players as possible in the next few years provided they are good enough to play for the African champions.
George Lebese was also signed from Kaizer Chiefs last week, as Mosimane continued to bolster his squad ahead of what is expected to be another grueling campaign.
It is unclear at this stage whether or not Sekotlong was registered with Caf for the knockout stages, but both Lebese and Oupa Manyisa are in the team's Caf Champions League squad.