Razak failed to make an impact in his single season at Sundowns in 2017/18, registering six appearances before the former Cordoba man was released by the Absa Premiership champions.
Downs have since recruited Pieterse from SuperSport United to provide competition to first-choice keeper Denis Onyango alongside Zambian veteran Kennedy Mweene. Thela Ngobeni's future remains uncertain at Chloorkop following his return from a loan spell at Free State Stars.
"Razak has left the club and that's why we brought in Reyaad Pieterse to strengthen our goalkeeping department," said Mosimane after his side started their title defence with a 1-1 draw against Kaizer Chiefs at Loftus Versfeld on Saturday.
The Brazilians on Friday also confirmed the signing of left-back Mosa Lebusa from Ajax Cape Town, having previously added Lebohang Maboe, Lyle Lakay, Andile Jali, Toni Silva and Jose Ali Meza to their first team squad.
Meamwhile, attackers Yannick Zakri and Keletso Makgalwa have reportedly been loaned to Maritzburg United.