Dak Prescott's foot-in-mouth moment following the Cowboys' loss on Sunday just cost him serious cash ... 'cause the NFL announced Thursday it's fining the Dallas QB $25K after he praised fans for throwing trash at refs.

Following a controversial end to the Cowboys vs. 49ers game at AT&T Stadium ... fans hurled garbage at officials as they exited the field of play.

One bottle actually hit Cowboys star pass rusher Demarcus Lawrence ... and when Dak was informed that the debris-throwing was actually aimed at umps, he commended the acts.