Merriman was arrested after Tequila signed a citizen's arrest warrant accusing Merriman of battery and false imprisonment. Both are felonies.
Dumanis decided not to charge Merriman after her office spent three days reviewing reports from the San Diego County Sheriff's Department.
"After a thorough review of the investigation into the Sept. 6, 2009 incident at the Poway home of Shawne Merriman, our office has determined there is insufficient evidence to prove beyond a reasonable doubt that any crime was committed," Dumanis said in a statement. "This case is now closed. No further comment will be made."
Dumanis' statement was released moments after the Chargers concluded practice. Informed by reporters that he wouldn't be charged, Merriman said he couldn't comment until after he'd spoken with his lawyers.
The Chargers are preparing for their season opener Monday night at Oakland. It will be Merriman's first game back since having knee surgery after the 2008 season opener.
Tequila, whose real name is Tila Nguyen, is best known for A Shot at Love with Tila Tequila, which ran for two seasons on MTV. The bisexual dating show featured men and women vying for Tequila's affections. She has also modeled for Playboy and other men's magazines.
Merriman said he was trying to keep Tequila from leaving his home because she appeared to be intoxicated and denied harming her.
Tequila's attorney, Alan Gutman, called Merriman's explanation "spin" and predicted it would be discredited, saying the 6-foot-4, 265-pound linebacker "brutally" assaulted his client.
Gutman did not immediately return calls seeking comment.