WME-ING is a sports, fashion and entertainment marketing company based in Beverly Hills that represents the likes of Ben Affleck, Martin Scorsese and Serena Williams.
The deal values the company at $5.5 billion and gives SoftBank an 8% stake.
WME acquired ING in 2014 for $2.4 billion.
SoftBank has been investing heavily outside Japan as it looks to create a separate international business, it has also invested in the likes of Legendary Pictures.