The prosecutor's office in the south Paris suburb of Evry said the incident occurred last Wednesday when the 30-year-old man, visibly drunk, boarded the bus and spotted a 21-year-old woman.
He slapped her on the buttocks, called her a "whore" and referred to the size of her breasts, comments which prompted the court on Friday to fine him 300 euros ($350) under the anti-harassment law passed last month.
He was also sentenced to three months in jail for physical abuse.
The woman managed to alert the driver who locked the bus's doors until the police arrived, enabling the offender's arrest.
French women have increasingly been speaking up about sexual assault and harassment in the wake of the MeToo movement.
"This is the first conviction for sexist insults in the country," a justice ministry spokesperson confirmed to AFP.
The new law also allows for on-the-spot fines for behaviour including comments on a woman's looks or clothing, catcalling, intrusive questions, unwanted following and "upskirting" — taking pictures under a woman's dress without her knowing.
Equality Minister Marlene Schiappa, who steered the law through parliament, hailed the decision and the driver's quick thinking.
"Together we will end sexist and sexual violence," she posted on Twitter.
Reported cases of sexual harassment and violence have surged this year, with nearly 28,000 complaints registered by police in the first seven months of 2018, up 23 percent from the period a year earlier, according to the interior ministry.
In one of the more shocking recent cases, a woman posted a video in July showing her being punched in broad daylight by a man outside a cafe after she angrily responded to harassing comments.
Photo: Jean Francois Gornet/Flickr