Marta, who is her country's all-time leading scorer and will retire from international football this year, had been shown a straight red card in their group loss to Spain on July 31st after a dangerous challenge late in the first half.
Brazil had appealed the two-match suspension to CAS, hoping to reduce her ban to just one game and be eligible for Tuesday evening's semi-final against the Spaniards.
Marta already sat out her team's quarter-final against France on August 3rd.
"The Applicants sought the annulment of the Challenged Decision and a new decision establishing that the player serves only a one-match suspension," CAS said.
"In view of the urgency, she issued her decision this afternoon, rejecting the application," CAS said.
Brazil face Spain at 21:00 CET on Tuesday night - follow all the action with Flashscore.