Garcia was an all-pro selection four times. He played for San Francisco, Cleveland, Detroit, Tampa Bay and had two stints with the Philadelphia Eagles.
The UFL is in its second season and Omaha is a first-year franchise.
"I am so ready and excited to be heading to the great city of Omaha to represent the Nighthawks of the United Football League," the 40-year-old quarterback said in a statement.
"I know that the state of Nebraska has a great passion for football and I can already see that the people of Omaha have fully embraced the Nighthawks. I still have the drive, commitment, and ability to play this game at the highest level and one thing that I definitely share with the people of Omaha is a passion for the game. I look forward to sharing these future experiences with the Nighthawks' great fans."
Garcia, who appeared in one game with the Eagles last season, threw for 25,537 yards, 161 touchdowns and 83 interceptions in 125 career NFL games.
The San Jose State product also starred in the Canadian Football League.