The J.J. Watt charity train keeps on rollin' ... as his Hurricane Harvey relief fund just broke $10 MILLION.

And a couple HUGE celebs helped him do it -- Ellen DeGeneres and Miley Cyrus!!

Ellen presented a check from Walmart for a cool $1 mil during a show taping on Wednesday -- adding to her previous personal donation of $50K.

Miley threw down $500K of her own ... and was pretty emotional as she discussed the tragedy in H-Town.

J.J., who FaceTimed into the show, said "I have no words" ... before promising their cash would go a long way toward helping victims.