Johnson had only been issued a yellow card for the incident (video) that the MLS Disciplinary Committee deemed to be violent conduct in the 30th minute of the game RSL won, 3-0.
Johnson, who did not appeal the suspension, got into a Twitter battle with D.C. fans and later apologized for insulting them.
Johnson, who will miss matches against the Colorado Rapids and Sporting Kansas City, was suspended earlier in the season for recklessly challenging Montreal goalkeeper Troy Perkins.
The latest decision leaves Johnson 1-2 on the MLS discipline front this season. He had his red card picked up in D.C. United's 2-1 loss to New England rescinded by MLS's independent review panel in May.