The Texas-based company, SCA Promotions, wants Armstrong to repay money the firm insured from his Tour victories in 2002, 2003 and 2004, after he was stripped of his record seven Tour titles last year.
"It is time now for Mr Armstrong to face the consequences of his actions," SCA's complaint said.
"This includes returning all of the funds paid to him by SCA, which totals more than $12 million."
"Mr. Armstrong has no legal right to retain any prize money paid to him by SCA because he is not the official winner of any Tour de France titles," it said.
The filing in a Texas state court not only could cost Armstrong financially but could force him to testify under oath about the doping scandal that ruined his cancer-comeback story and tainted his Livestrong Foundation charity work.
Armstrong was also banned for life when the US Anti-Doping Agency (USADA) found overwhelming evidence that he was at the heart of a sophisticated doping scheme when his US Postal Service team dominated the Tour de France.
"Lance Armstrong perpetuated what may well be the most outrageous, cold-hearted and elaborate lie in the history of sports," the lawsuit said.
After years of denials, Armstrong confessed last month that he had taken performance-enhancing drugs in sweeping Tour titles from 1999-2005.
Armstrong sued SCA and won after the company delayed his 2005 bonus payment because of reports in Europe that the American had used performance-enhancing drugs.
"We think there are several avenues for us to seek recovery on this," SCA attorney Jeff Dorough told AFP.
"Armstrong and his lawyers said flat-out at that time that if he was ever stripped of the titles, they would pay the money back," Dorough said. "We're just seeking to hold them to their promises."
Armstrong attorney Mark Fabiani cited details of a 2006 settlement agreement between SCA and Armstrong in saying that the firm had no recourse to reclaim the bonus money.
"We are going to let the settlement agreement speak for itself. It is very clear on this point," Fabiani said. "The language of the agreement clearly bars SCA from wriggling out of the agreement."
Armstrong also faces a lawsuit from former US Postal teammate Floyd Landis, who was stripped of the 2006 Tour de France title for doping but says Armstrong violated terms of a sponsorship deal with US Postal by using banned drugs.
ABC News has reported that federal agents are investigating Armstrong for crimes including obstruction of justice, as well as witness tampering and intimidation.
On Wednesday, USADA chief Travis Tygart extended a deadline that gives Armstrong until February 20 to cooperate with anti-doping authorities by testifying under oath about his activities to have any hope of seeing his ban reduced so he could compete in sanctioned cycling and triathlon events.