Hurricane Harvey is thankfully calming down ... but J.J. Watt's relief fund for Houston is still goin' strong -- pushing past the $20 MILLION mark on Tuesday. 

J.J.'s crowdfunding campaign started off with a $200K goal a little over a week ago ... and quickly ran up into the multi-millions with big-name celebs like Miley Cyrus ($500K), Drake ($200K) and Chris Paul ($75K) leading the way.

In case you're wondering how things are going ... Watt shared an update on social media after breaking $20 mil -- saying, "It has truly been an incredible spectacle to watch."