11:38 AM PT -- TMZ Sports spoke with Montgomery ... and she elaborated on her decision to sit out ... calling it a "leap of faith."
"I'm not breaking up with basketball, it's just pausing things because, right now, we can all see something's happening."
"Something's happening in America, something's happening all over. So, I wanna be a part of that something."
Montgomery added ... "It's just not about basketball right now. The people that are gonna go play that's fine, but for me, my heart is somewhere else. So, that's where I'm gonna go."
WNBA star Renee Montgomery will NOT be taking the court when the 2020 season returns in July ... saying she's going to use her time to fight for social justice instead.
Montgomery -- the 4th overall pick out of UConn in 2009 -- made the announcement on Twitter early Thursday ... deciding her services are needed elsewhere.
"After much thought, I’ve decided to opt out of the 2020 WNBA season," the Atlanta Dream guard said. "There’s work to be done off the court in so many areas in our community."
"Social justice reform isn’t going to happen overnight but I do feel that now is the time and Moments equal Momentum. Lets keep it going!"
Montgomery joins superstar Maya Moore, who temporarily left the game to help fight for justice for Jonathan Irons ... a 39-year-old man she believes is wrongfully convicted of burglary and assaulting a homeowner with a gun when he was 16.
Dream coach Nicki Collen released a statement supporting Montgomery's decision, saying, "While I am saddened Renee will not be in a Dream uniform this summer, I am incredibly proud of her passion for her foundation, her outreach in the community and her chance to impact the Black Lives Matter movement with her platform as a WNBA athlete."