The government had previously announced it would join the whistle-blower lawsuit brought by former Armstrong teammate Floyd Landis. Tuesday was the deadline to file its formal complaint.
The Postal Service paid about $40 million to be the title sponsor of Armstrong's teams for six of his seven Tour de France victories. The filing says the USPS paid Armstrong $17 million form 1998-2004.
An Armstrong attorney says the Postal Service made money off the cyclist and called the federal complaint "opportunistic and insincere."
Armstrong admitted in January to using performance-enhancing drugs to win.
The lawsuit also names former team Armstrong team director Johan Bruyneel and team management company Tailwind Sports as defendants.