LeMond, a three-time winner of the Tour de France, told The Denver Post he was glad to get a subpoena to testify in federal court in Los Angeles July 30 in a probe of possible fraud and doping by Armstrong and his associates.
"The evidence will come from the investigation," LeMond told the newspaper, "and I believe it will be overwhelming."
LeMond said he believes disgraced Tour winner Floyd Landis told the truth when he described Armstrong's systemic doping program.
"I think he's telling the truth," LeMond told the newspaper. "I think the level of detail, the descriptions, I think it rings true."
Armstrong, who won the French race seven straight times, has denied allegations of doping. On Monday, his attorney sent a letter to the federal prosecutor looking into the case complaining about leaks to the media.
"This appears to be a full-blown and largely unmonitored exploration of Floyd Landis' patently unreliable and routinely changing accusations of possible improper conduct in the professional cycling industry at large," wrote Armstrong's attorney, Tim Herman.
"I'm hoping it gets as far as it can," LeMond told The Denver Post.
LeMond's belief in Landis' allegations is quite a turnaround from three years ago, when LeMond testified in Landis' doping case. On a remarkable day in that hearing, LeMond told how Landis' manager threatened to reveal then-secret stories of LeMond being sexually abused as a child if he came to the hearing.