The owner of the Houston Rockets just pulled out his checkbook -- AGAIN -- for the victims of Hurricane Harvey ... upping his original $4 million donation to a whopping $10 MILLION.

Leslie Alexander told Houston mayor, Sylvester Turner, that he is more than doubling his initial contribution to the city on Tuesday ... according to Jonathan Feigen of the Houston Chronicle.

The Rockets released a statement after Alexander's initial donation, saying "our hearts are heavy seeing the devastation that so many of our friends, family and neighbors are experiencing."