The Mohegan Tribe Department of Athletic Regulation told ESPN that Newton had been banned and fined after testing positive for the drug.
"It is important for all athletes to abide by the rules of the governing commission and we fully support the commission in their decision," Bellator said in a statement. "We look forward to seeing Mr. Newton get back in the cage once he serves his suspension."
Newton, who is widely regarded as one of the best light heavyweight fighters outside the UFC, lost his title to Liam McGeary on February 27 via unanimous decision at Bellator 134 in Uncasville, Connecticut.
Newton has the right to appeal against his suspension and fine.