James Harden is being sued by a man who claims the NBA star LIED to his face about how he intended to use a MASSIVE $30 mil Beverly Hills mansion … claiming The Beard turned his spot into a party house!

The man behind the suit is George Santopietro who owns a 14,500-square-foot estate in the heart of Beverly Hills … 8 bedrooms, 10 baths, gym, spa, wine cellar, pool, etc.

In other words, the place is INSANE!!!

Santopietro claims Harden signed a contract to use the home as a vacation rental for 1 week starting August 11, 2019 … paying $82,200 for the stay.

But, Santopietro says there was specific language in the rental contract saying Harden could not have any more than 7 adults on the premises at any one time.