Son joined Tottenham in 2015 from Bayer Leverkusen and has made 454 appearances, scoring 173 goals.
The 33-year-old is set to join Los Angeles FC in the MLS, where negotiations are at an advanced stage.
Son made the announcement at a media conference in Seoul ahead of Tottenham's preseason friendly against Newcastle.
He explained that leaving the club was a difficult decision, but he needed a new environment to push himself and experience a change after ten years.
Son expressed gratitude to Tottenham fans for their love and support, and hopes for a respectful goodbye.
Spurs manager Thomas Frank praised Son as a true Tottenham legend and one of the greatest players in the Premier League.
