ICYMI -- Moss opened up about his time with RG3 and the Redskins on Monday ... saying the Heisman-winning QB gloated about Shanahan presumably getting canned over his lack of playing time.

But RG3 says the story is complete B.S. ... and just responded to Moss on Twitter "No subtweeting needed. Santana Moss, I treat you like a brother & have always had your back. To openly lie about me is a betrayal....."

"Some so desperately want me to fit this negative narrative that has been pushed about me. But I don't fit it. Never have. Never will."

"Proved it in Cleveland. Voted Captain. Came back to play for my teammates just to help us win 1 game. With a broken shoulder. Stop the lies."

There's a lot more in Moss' interview with 'Dukes vs. The World' on 106.7 The Fan ... he hints that RG3's current unemployment is karma for the way he acted in Washington.