The league confirmed the two-game suspension in a statement, saying Bell also will be held without two game checks for violating its substance-abuse policy.
Bell was suspended in April for his August 2014 arrest on DUI and marijuana possession charges. Bell and then-teammate LeGarrette Blount were arrested while driving to the airport for a preseason game.
At the time of Bell's appeal, his attorneys and the union argued that he was being punished under the league's new drug policy for an offense that occurred under the old policy. The arbitrator encouraged the sides to work toward a settlement.
"As I have stated before, we were disappointed in Le'Veon Bell's actions last August," Steelers general manager Kevin Colbert said in a statement Tuesday. "Le'Veon made a mistake and now he must learn from his mistake and focus on eliminating distractions from his life. We look forward to continuing to work with Le'Veon to try to help him reach his full potential as a person and as a player."
Blount, who now plays for the New England Patriots, was charged with misdemeanor marijuana possession and has been suspended for the first game of the 2015 season. Bell initially was suspended an additional two games for the DUI charge.
In addition to the NFL penalties, Blount was sentenced to 15 months' probation and attended a pretrial intervention program. His driver's license also was suspended for 60 days.
Bell remains eligible to take part in training camp and preseason games. The reduced suspension means he will miss the NFL season opener against the Patriots and the Steelers' Week 2 matchup against the San Francisco 49ers. He is eligible to rejoin the team Sept. 21.
On Saturday, Bell said he "most definitely" wanted his suspension reduced but said he had no idea how the league would handle his appeal or whether it was even up for evaluation.
"I want to make sure I do everything in my power to be a better teammate and player," Bell said on the first day of Steelers training camp. "Whatever the consequence is, I'll take it. ... You have to take the good with the bad."
Last season, Bell was second in the NFL in rushing yards (1,361) and receptions for a running back (83).
Information from ESPN.com's Jeremy Fowler was used in this report.