BIG WIN for Conor McGregor ... the UFC star got his big $75,000 fine reduced to $25,000 for that water bottle fight with Nate Diaz at the UFC 202 presser.

Remember, the Nevada State Athletic Commission hit McGregor with the massive fine -- plus 50 hours of community service. Conor was PISSED!!! So was Dana White.

But the Commission granted McGregor a rehearing which took place moments ago ... and agreed to REDUCE the punishment to a $25k fine plus 25 hours of community service, per Damon Martin of FOX Sports. He has 6 months to complete the community service. 

And get this ... now the Commission is recommending a rehearing for Nate Diaz as well, who got the same initial punishment as Conor.