Ntamack was punished for the red card he received following a dangerous tackle on opposite number Ben Thomas in his team's 43-0 rout of Wales in Paris last Friday.
He will miss Saturday's clash with England at Twickenham but will be available for selection to face Italy in Rome on February 23.
Ntamack, 25, and playing in his first Test since August 2023, was initially given just a yellow card for his tackle on Thomas but that was upgraded to red. It was the fly-half's first red card of his career.
He could have been hit with a six-week ban but the Six Nations disciplinary committee ruled that his "exemplary disciplinary record" allowed for the sanction to be halved.
Ntamack's place in the team for Twickenham is likely to be filled by Matthieu Jalibert.