According to the New York Times, the former Newcastle United and Al-Ahli Saudi winger grabbed a sensitive area of his body while exchanging words with Orlando's Wilder Cartagena, with Charlotte leading 3-0.
Referee Victor Rivas missed the incident at first. Video assistant referee Kevin Terry Jr sent him to the pitchside monitor, and after the review, Rivas brandished the red card at Saint-Maximin.
The laws of the International Football Association Board (IFAB) state that "using offensive, insulting or abusive language or action(s)" is an offence that warrants a red card.
Saint-Maximin had scored Charlotte's second goal in the 49th minute. The French winger then stepped up for a penalty in the 84th, only for Orlando goalkeeper Maxime Crepeau to save it, with Abdoulaye Doucoure following up to score. Charlotte ran out 4-0 winners.
One game of the regular season remains for Charlotte, against Inter Miami next Saturday. The team sit fourth in the Eastern Conference and have already booked their play-off spot.
Since arriving from France's Lens last July, Saint-Maximin has proved an important figure for Charlotte, scoring two goals and providing four assists in 14 MLS matches.
Read also: Video: glaring refereeing error in the Milan and Inter derby
Read also: A cinematic ending frightens him: Amorim warns Milan despite beating Inter