The forward has signed a contract with the Club until 2020.
The South Korea international originally moved to Germany as a teenager and came through the ranks at Hamburger SV, scoring on his Bundesliga debut in October 2010 against 1. FC Köln.
A consistent goalscorer throughout his time in Germany, Son went on to make 78 appearances and score 20 times for Hamburg before joining Bayer Leverkusen in June, 2013.
He has helped them to Champions League qualification in each of his two seasons and made a total of 87 appearances, scoring 29 times at BayArena.
The 23-year-old is also a regular at international level with 11 goals in his 44 appearances for South Korea to date, including scoring against Algeria at the 2014 World Cup Finals.
He will wear the number 7 shirt with ‘Son’ on his back.