An additional $50 million or so will be available for teams to spend. The official salary cap has not been determined, but before the ruling it was expected to rise by at least $10 million from the $143.5 million ceiling of last season.
The arbitrator's ruling was made last week but not announced until Tuesday.
The NFL calls the adjustment a technical accounting matter. The NFL Players Association contends the league miscalculated or was hiding money due the players.
"Our union will always look to enforce our economic rights under the collective bargaining agreement," the NFLPA said in a statement.
The issue was whether specific revenue the league called exempt from being applied toward the salary cap actually should be counted.
The NFL said the decision will "increase the number of seasons over which the league's investments in stadium projects will be recouped from local stadium revenues."
"The decision should have no effect on the league's long-standing commitment to stadium improvements," the league added. "That commitment was most recently reflected in the league's decision to provide hundreds of millions of dollars in financial support for the Rams' new stadium in Los Angeles as well as financial support for new stadiums in Oakland and San Diego."
The union calculated the money excluded exceeded $100 million over three years.
A new salary cap figure will be announced before the NFL's business year begins March 9. If it increases by more than $11 million, it would be the biggest rise since 2006, when the cap went from $85.5 million to $102 million.