The centre-back joins the Brazilians as a free agent – he last played for Maritzburg United.
Onyango joined the Pretoria side to help the side to defend their league title under a new coaching structure of joined head coaches Manqoba Mngqithi and Rulani Mokwena. The head coaching duo is assisted by former Kaizer Chiefs coach Steve Komphela.
The 26-year-old has played in top-flight football in South Africa for over three seasons with the Team of Choice in KZN following a successful stint Santos in the lower league.
Onyango will face stiff competition for a place in the starting line up with the technical staff selecting Motjeka Madisha and Rivaldo Coetzee at centre-back for the first two games of the 2019/2020 Dstve Premiership season.
“Brian Onyango has joined Mamelodi Sundowns on a three-year deal with a two-year option to extend,” read the Sundowns website.
“The centre-back comes with a wealth of international and local experience as he has played for Santos, the Team Of Choice and his national side. Onyango first moved to South Africa in 2012 to join the now ABC Motsepe based Santos. Welcome to Masandawana Brian Onyango!”