Billie Eilish Dropping Mitchell & Ness NBA Hat Collection with Fanatics, Complex

Billie Eilish is putting her stylish stamp on the NBA -- the singer is giving fans a chance to rock her signature look at games ... by releasing her own team hats!!

TMZ Sports is told the nine-time Grammy winner teamed up with Fanatics and Complex to release some limited-edition Mitchell & Ness lids -- each being decked out in custom Billie Eilish patches to highlight "authenticity, nostalgia and individuality."

The "BIRDS OF A FEATHER" crooner is no stranger to rocking M&N attire -- she loves her throwback jerseys ... and usually accessorizes the threads with a snapback or fitted whenever she hits the stage.

Now, hoop fans can show off their favorite musician and basketball squad at the same…