The man placed the stone, which has marked the border for more than 200 years, 2.29 metres into France after it got in the way of his tractor.
The slight boundary change was noticed several weeks ago by an amateur historian during a walk in the forest.
The farmer “also repositioned his fence on trees that belong to the wood of Bousignies”, according to Aurélie Welonek, the mayor of the French town Bousignies-sur-Roc.
The stone was first put there in 1819, before the 620-km (390-mile) border was officially established under the 1820 Treaty of Kortrijk.
David Lavaux, the mayor of the Belgian village of Erquelinnes, said the farmer will have to move the stone back or could face criminal charges.
“We have no interest in expanding the town, or the country. He made Belgium bigger and France smaller. It’s not a good idea,” he told French TV channel TF1.
He added: “I was happy, my town was bigger. But the mayor of Bousignies-sur-Roc didn’t agree.
“If [the farmer] shows goodwill, he won’t have a problem, we will settle this issue amicably,” Lavaux said with a smile.
Ms Welonek agreed, telling La Voix du Nord: “We should be able to avoid a new border war.”
If the boundary stone is not returned to its usual spot, a Franco-Belgium commission could be established to determine the exact border.
The last commission of this kind took place more than 90 years ago, according to Mr Lavaux.