PARIS — A French court on Thursday convicted and fined a Tour de France spectator whose cardboard sign caused a pileup of dozens of cyclists during the world-famous competition in June.

The spectator, a 31-year-old woman who has not been publicly identified, was found guilty by a criminal court in Brest of reckless endangerment and involuntarily causing injuries.

The woman, who stepped onto the road during the race and had her back to the cyclists, was fined 1,200 euros, or about $1,350 — far less than the maximum penalties she was facing.

The charges under consideration carried a possible one-year prison term and a fine of €15,000. At the woman’s trial, in October, prosecutors requested a four-month suspended prison sentence. She did not have a previous criminal record.

The crash occurred on June 26 in the Finistère area of Brittany, in western France, less than 30 miles from the finish line in the first of the race’s 21 stages.

The woman, who wore a bright yellow jacket, was facing television cameras and holding a piece of cardboard bearing an affectionate message for her grandparents — longtime fans of the Tour who were watching from Paris — when a German rider, Tony Martin, hit the sign.

Mr. Martin fell to the ground, setting off a cascade of collisions in the middle of the main cluster of cyclists zooming by. Several injured riders were forced to drop out of the competition.

Video of the pileup quickly went viral, focusing intense media scrutiny on the woman, who left the scene before the police could reach her. Prosecutors opened an investigation, and the woman turned herself in four days later.

Crashes caused by reckless spectators or vehicles are not uncommon during professional cycling competitions, in which thousands of cheering fans line roads right next to the cyclists.

During the 2016 Tour, spectators who blocked a television motorcycle created such chaos that the holder of the race leader’s yellow jersey had to briefly head up a mountain pass on foot to try to maintain his lead.