The suspension comes as a result of an investigation that started more than a year ago after an ex-girlfriend accused Elliott of domestic violence in Columbus, Ohio. The Columbus City Attorney's office announced in September that it would not pursue charges against Elliott because of "conflicting and inconsistent information," but the NFL can penalize a player even without legal charges.
Elliott said in a statement Friday evening that he "strongly" disagrees with the league's conclusion.
Elliott is planning to appeal the suspension, his representatives said in a statement.
"The NFL's findings are replete with factual inaccuracies and erroneous conclusions and it 'cherry picks' so called evidence to support its conclusion while ignoring other critical evidence," Frank Salzano and Scott Rosenblum said in the statement.
Elliott has three business days to file notice of appeal, and a hearing must be scheduled within 10 days of receipt of the notice, according to Article 46 of the collective bargaining agreement, which governs appeal of commissioner discipline.
Commissioner Roger Goodell or his designee, usually former NFL executive Harold Henderson, would hear the appeal, which would be scheduled for the second Tuesday after the receipt of the appeal. Any ruling by Goodell or his designee would be binding.
Without an appeal, Elliott's suspension would begin Sept. 2. He would be eligible to return to the active roster Oct. 23, the day after the Cowboys' Week 7 game against the San Francisco 49ers. His first possible game would be Week 8 at Washington.
The NFL Players Association said in a statement that it is reviewing the league's decision and has been in contact with Elliott and his representatives to consider their options.
An NFL official told ESPN that the decision on punishment for Elliott took time to make in part because the NFLPA delayed delivering requested information from mid-December to mid-May.
However, NFLPA president Eric Winston expressed frustration with the disciplinary process Friday night, saying, "There's just a lack of a protocol, there's a lack of a coherent process, there's a lack of anything, there's a lack of player buy in, because [the league has] chosen to go at it alone."
Legal authorities concluded that they could not prove Elliott's guilt beyond a reasonable doubt. Several witnesses, according to sources close to the case, had heard the woman yell at Elliott that she would "ruin his career" when she was unhappy that they would not continue their relationship.
The league's suspension of Elliott differs from the 2014 ban of then-Baltimore Ravens running back Ray Rice, who was prosecuted and agreed to a plea resolution after a full investigation by legal authorities.
NFL officials said in a statement that it investigated Elliott's case thoroughly and determined that he violated the personal conduct policy. The league examined text messages, interviewed more than a dozen witnesses -- including the accuser and Elliott -- and retained medical and legal experts to help Goodell make his decision.
In a letter to Elliott advising him of the league's decision, the NFL said the experts "were of the view that there is substantial and persuasive evidence supporting a finding that [Elliott] engaged in physical violence" multiple times in July 2016.
The NFL identified three incidents that caused injuries to the woman's face, neck, shoulders, arms, hands, wrists, hips and knees over the span of five days and cited photographic evidence of the injuries.
Peter Harvey, the former attorney general for New Jersey who aided the NFL's investigation, said the league analyzed metadata related to the photographs and was able to determine the photos were taken on the date the woman said the violence occurred.
Harvey said Elliott's representatives presented "alternative theories" to how she may have sustained the injuries, including a fight with another woman, falling down the stairs or at her job as a server, but did not offer evidence of the injuries occurring under those circumstances.
"So while alternative theories are interesting, in my judgment they have to be supported by evidence, and that was lacking in this particular situation," Harvey said.
Elliott's representatives said in their statement that the prosecutor's office and NFL investigators told them that the woman had lied about one of the alleged incidents and that a witness had stated that no assault had occurred. They also challenged the timing of her injuries, saying they happened when Elliott and the woman were not together.
"During the upcoming weeks and through the appeal, a slew of additional credible and controverting evidence will come to light," they said.
The league has also ordered Elliott to arrange a clinical evaluation and to comply with any recommendations for counseling or treatment.
Cowboys owner and general manager Jerry Jones has defended Elliott throughout the NFL's investigation, and a source told ESPN NFL Insider Adam Schefter that Jones is furious with the NFL's decision.
A Cowboys source told ESPN's Josina Anderson that the team is standing behind Elliott after the announcement of his suspension.
Before disciplining Elliott, the NFL considered the domestic violence allegations from February 2016 and July 2016, as well as a St. Patrick's Day incident this March in which he pulled down a woman's shirt, although that did not factor into the league's decision on discipline.
NFL vice president of communications Joe Lockhart said Friday that the league told Elliott he used "poor judgment and questionable behavior" in pulling down the woman's top but that the action did not rise to the level of adding to his penalty.
As to Jones' comments that this case did not have "anything to do with domestic violence," Lockhart said, "I don't think we have a comment one way or the other on anything that anyone else in this case, involved in this case or that has looked at this case or commented on this case. I can say that we believe this investigation was thorough, exhaustive and fair to all parties involved."
Elliott is the third Cowboys player who will miss time this season because of a suspension. Defensive end Randy Gregory has been suspended for the season after multiple violations of the substance abuse policy. Defensive end David Irving has been suspended the first four games for violating the performance-enhancing drug policy.
There are people in and around the Cowboys organization growing increasingly concerned about Elliott's lifestyle and choices, sources close to the situation told Schefter. Elliott has become known for frequenting Dallas nightclubs, and there are concerns about the way he has conducted himself, the sources said.
Before the team left for training camp, Elliott was allegedly involved in an incident at a Dallas bar that left a man with a nose injury. Dallas police have suspended the investigation because they could not contact the victim and no witnesses came forward.
Pending his appeal, Elliott will not be allowed into the Cowboys' training complex from the first week of the regular season until the suspension is complete.
Elliott, the fourth overall pick in the 2016 draft, led the NFL in rushing last season with 1,631 yards on 322 carries. He scored 15 rushing touchdowns. He also caught 32 passes for 363 yards and one touchdown and was named to the Pro Bowl.
With Elliott out, the Cowboys will turn to Darren McFadden, Rod Smith and Alfred Morris as their running backs. The team signed veteran Ronnie Hillman at the end of July, but coach Jason Garrett said the addition had nothing to do with Elliott's potential suspension.
McFadden finished fourth in the NFL in rushing in 2015 with 1,089 yards despite not taking over until the seventh game of the season. He was limited to 24 carries last year because of a broken elbow. Morris has three 1,000-yard seasons to his credit but had 69 carries for 243 yards and two touchdowns in limited work last season. Smith has been taking turns as the third tailback in the early part of training camp.
The suspension will be costly for Elliott. In addition to losing his base-salary pay, the guarantees in Elliott's $24.96 million deal will be voided. He received a signing bonus of $16.35 million and got the final deferred installment of the bonus in March. Once he misses games, he will have to pay back some of the signing-bonus money. For each game missed, Elliott will have to repay about $240,000.
Elliott's base salaries for 2017 to 2019 are worth $1.58 million, $2.72 million and $3.85 million, respectively. He would still receive those amounts for as long as he is on the roster, but they would no longer be guaranteed if an injury ended his career.
Information from The Associated Press was used in this report.