The Brazilians have continued their mid-year shopping spree ahead of the 2018/19 season.
Pieterse joins from cross-town rivals SuperSport United, where he spent the last two seasons after exiting Kaizer Chiefs.
Meanwhile, Lebusa arrives after having his exit from Ajax Cape Town confirmed earlier on Friday.
The duo arrive just hours before Sundowns are set to begin their Absa Premiership title defence against Soweto giants Kaizer Chiefs at the Loftus Versfeld Stadium on Saturday.
The former CAF Champions League champions have now brought in a total of seven new first team signings; the others being Lebohang Maboe, Lyle Lakay, Andile Jali, Toni Silva and Jose Ali Meza.