Russell Simmons -- I Did NOT Hijack Millions From Ex-Biz Partner!

Russell Simmons is firing back at his ex-business partner's legal threats ... telling TMZ he did NOT siphon off millions of dollars from his sneaker company -- and any accusations otherwise are the workings of a "disgruntled" employee.

TMZ broke the story ... former Run Athletics President Rasheed Young threatened to sue Russell and Reverend Run ... claiming the hip-hop moguls screwed him out of millions, by secretly funneling revenue from their company into a separate account.

Russell was out in L.A. Monday and tells TMZ ... any accusations of hiding profits are 100% "not true".

He continues, "[Rasheed] worked for me ... I fought to keep his job ... and I'm sorry he's disgruntled, but that's what happens. Anyone can say anything if a celebrity's name's attached to it ... there's nothing I can do about it ... it's false."