The league confirmed the additional punishment in a text message to ESPN's Adam Schefter on Friday morning.
Brown served a one-game suspension at the start of the 2016 season after domestic violence allegations were made by his ex-wife. He accepted the additional ban following further investigation by the NFL, according to ESPN.
"We reopened the investigation based on new info," the league texted to ESPN. "Concluded there was a violation of our personal conduct policy and imposed 6 game suspension which he accepted without appeal."
The new ban is mostly a procedural move since the 38-year-old Brown hasn't kicked in the NFL since the Giants released him last October.
The timing of the announced decision is interesting as it comes on the same day that Dallas Cowboys running back Ezekiel Elliott is waiting to hear from a U.S. District Court judge in Texas whether he will be granted a temporary restraining order that would allow him to continue to play this season.
Elliott is eligible to play in Sunday night's season opener against the New York Giants even though an arbitrator on Tuesday upheld his six-game suspension handed down by NFL commissioner Roger Goodell on Aug. 11. The ban stems from the league's 13 month-long investigation into domestic violence accusations made against Elliott by a former girlfriend.
"Timing has nothing to do with Zeke and made no sense to hold this until Zeke was complete," the league texted to ESPN.
Brown will start serving the six-game suspension immediately even though he remains unsigned, a source told ESPN.
Brown was placed on the exempt list in October when the NFL decided it wished to further investigate his May 22, 2015, arrest in Woodinville, Wash., on suspicion of domestic assault in the fourth degree. The Giants took it a step further by cutting ties with Brown just four days later on Oct. 25.
Charges were never filed in the case, but documents were released last October related to Brown's arrest. The letters, emails and journals contained admissions by Brown that he had physically, verbally and emotionally abused his wife.
Brown told "Good Morning America" in February that he never hit Molly Brown, his now ex-wife.
"I mean, I had put my hands on her. I kicked the chair. I held her down. The holding down was the worst moment in our marriage," Brown said during the interview. "I never hit her. I never slapped her. I never choked her. I never did those types of things.
"These were the things that you say to yourself and then you'd burn them. ... And I didn't. The fact that my private things are being used against me, that's hard to swallow. I'm talking about my journals. I had to learn all that and write that down in order to heal, and now you're telling me that I'm going to be punished for trying to correct the things in my life that needed to be changed."
The Giants' decision to move on from Brown came just hours after the veteran released a statement in which he apologized for his actions. He said he was getting help for his troubles and denied his ex-wife's accusations of the more than 20 physical incidents.
The Giants signed Brown to a two-year, $4 million deal in April 2016 despite knowing he was under investigation for domestic abuse.