Tau leaves Mamelodi Sundowns after helping them win their eighth PSL title and will now be looking to make a mark abroad.
The 24-year-old attacker has signed a four-year deal with the club, but will be loaned out for the coming season.
Brighton manager Chris Hughton said: “Percy is a player we have highlighted for the future, and there is no doubting his talent, having been voted South African player of the season.
“Percy will go out on loan this season, play regularly and get some experience of European football.
“We will be watching his progress and monitoring his development over the season.”