Police and firefighters were called to the site of the tomb on Ile d'Yeu in western France at 4:00 am (0200 GMT) on reports of a trash bin fire at the Port-Joinville cemetery.
The fire was quickly extinguished, but police then discovered that the cross on Petain's tomb had been broken.
Some letters were also written on the tombstone but police were unsure of their significance.
In 2007, Petain's tomb was also vandalised with the white cross broken and trash dumped on the site.
Petain, a French military leader hailed as a hero of World War I, was the head of the government that capitulated to the Nazis and subsequently collaborated with their occupation of France, including the deportation of tens of thousands of Jews to death camps.
“It happens from time to time that the tomb is targeted and some people throw things on top of it,” local official Sylvie Groc told AFP.
“We avoid disclosing it so people don't get bad ideas,” she said, adding that now “we have to lock the cemetery at night”.
The incident comes the day before the anniversary of Petain's death on July 23, 1951, at the age of 95.
While the Nazis occupied the north of France, Petain led so-called Vichy France in the centre and the south of the country, with its headquarters in the eponymous spa city.
Despite having autonomy from German policies, Petain passed legislation that saw Jews — around 150,000 of whom had fled to southern France believing it to be safer — subjected to severe discrimination similar to that in the Nazi-occupied north.
Under Petain, the Vichy regime put to death up to 15,000 people and helped deport nearly 80,000.
After the war Petain was convicted of treason and condemned to death but General Charles de Gaulle commuted his sentence to life in prison.