Foster had been scheduled for arraignment Thursday in Hillsborough County.
Prosecutors concluded there was insufficient evidence to file charges against Foster after "a meticulous review of the facts of the case," said Estella Gray, director of communications for the State Attorney's Office.
She didn't elaborate further.
"We take allegations of domestic violence very seriously and thoroughly investigated this case," Gray said in an email to The Associated Press.
"We're disappointed. The prosecutor's choice is yet another slap in the face to Miss Ennis, just like the one Mr. Foster gave her in Tampa," Ennis' attorney, Adante Pointer, told ABC News. "Elissa Ennis was fully ready to participate and hold him accountable. This is not a situation where she recanted, nor that the prosecutor had new evidence that it didn't happen."
Ed Suarez, an attorney for Foster, declined to comment to ESPN.
Ennis, with whom Foster has been involved off and on for three years, previously discussed the incident during a December interview with "Good Morning America."
Foster was released by San Francisco in late November and claimed on waivers by Washington just three days after being arrested. The team received heavy criticism for picking up Foster. He remains on the commissioner's exempt list.
"We are monitoring all developments in the matter, which continues to be under review by the league," an NFL spokesman said Thursday.
Tampa police said Foster and Ennis were involved in a verbal altercation and that "Foster slapped her phone out of her hand, pushed her in the chest area, and slapped her with an open hand on the left side of her face." Police said a 1-inch scratch was observed on her left collarbone.
It wasn't their first domestic incident. Last February, Foster was arrested on suspicion of domestic violence and possession of an assault rifle, a loaded Sig Sauer 516 rifle. The domestic violence charge was dropped in May when Ennis recanted her allegation that Foster had hit her; Ennis told GMA that she lied when she recanted, saying she did so out of love.
Foster pleaded no contest to a misdemeanor weapons charge, received two years of probation and was banned from possessing guns during that time.
He served a two-game suspension to start the 2018 NFL season, stemming from an arrest for misdemeanor marijuana possession last January. Charges were dropped after he completed a first-time-offender diversion course.
ESPN's John Keim and The Associated Press contributed to this report.