The England international was cited after the Champions Cup match last weekend and his punishment will now see him miss two of the autumn internationals.
Marler will be absent for England's games against Argentina and Australia next month after admitting he hit Wasps lock Will Rowlands in the 41-10 loss.
The news will come as a blow to England coach Eddie Jones although Marler will have the chance to appeal the decision.
In a statement, tournament organizers European Professional Club Rugby said: "The committee upheld the citing complaint and determined that the offence had warranted a red card, but decided after consideration that the strike by the player was with the arm rather than the elbow.
"In deciding that Marler is suspended until midnight on Sunday, November 19, the Committee took into account the player's likely playing commitments in the coming weeks.
"Both Marler and EPCR have the right to appeal the decision."