The 24-year-old had been linked with a possible move to the Brazilians for a while, and he has now put pen to paper after leaving Rise and Shine.
He began his new journey at Chloorkop on Friday, and he was presented in the Sundowns shirt at the of the final training session at the club's headquarters.
Editors' Picks
The midfielder is seen as a long-term replacement for Hlompho Kekana who has been ever-present for the Brazilians over the past five seasons.
Seabi is a utility player who can play both as a defensive and offensive midfielder.