Reed was penalized for a hit to the head of Steelers receiver Emmanuel Sanders on Sunday, his second such violation this year and third in three seasons.
The previous ones came Sept. 23 against the Patriots and Dec. 19, 2010, against the Saints.
"We cannot tolerate repeated violations of rules, especially rules related to player safety," NFL executive vice president of football operations Ray Anderson said in a statement. "We will continue to take the strongest possible action to deter these types of violations and protect our players."
Ravens coach John Harbaugh said Reed will appeal the suspension immediately and that the league has told the team to expect a decision by Wednesday.
If Reed does not win his appeal, he will not be allowed to practice this week or play Sunday at the Chargers.
Harbaugh didn't expect the suspension and defended Reed.
"I was very surprised. I didn't see that one coming," Harbaugh said. "None of those hits, they were all inadvertent. None of those were intent to injure or to harm in any way. I think if you look at the hits, that's pretty obvious. We all know Ed. Ed respects the game, he respects his fellow players.
"He's a good person, he's got a good heart. He's got tremendous respect for the game and we stand behind him in that respect as a team and an organization."
If the Ravens have to play without Reed at San Diego, that would mark another hit to an already depleted secondary that is without cornerbacks Lardarius Webb and Jimmy Smith.
"It's like losing other guys," Harbaugh said. "It's going to be a factor. If we don't have Ed, that's hard. That'll be something that'll hurt us on defense. Once again, someone else is going to have to step up and play really well."