The 18-year-old has agreed a deal that will run until 2030 and will join the club in January 2025.
The winger broke through as a first-team regular last season at Gangwon FC – making 25 appearances this season, contributing eight goals and four assists.
He was given the division's Young Player of the Month award for April, May and June.
Yang has represented South Korea at under-16 and under-17 levels and is hotly tipped to break into the senior men's side soon.
He will join up with compatriot and current Tottenham captain Son Heung-min, with the youngster already being compared to the 32-year-old Korean football superstar.
Spurs boss Ange Postecoglou has a reputation for signing top players from Asia and bringing them to Europe, finding success with several transfers from the region during his time at Celtic in Scotland.
The north London outfit are currently on a pre-season tour in the continent, beginning in Japan and now playing fixtures in South Korea.
Spurs will play Team K League on Wednesday and then FC Bayern on Saturday – both at the Seoul World Cup Stadium.
