Georges Vandenbeusch, a Roman Catholic priest who moved to the area two years ago, had repeatedly ignored warnings by the French authorities that the region was dangerous.
France's foreign ministry said he was snatched overnight from his home near the town of Koza, about 30 kilometres from the Nigerian border.
He was kidnapped by around 15 people who burst into his Nguetchewe parish base without a car, said the bishop of the Nanterre diocese near Paris, which has authority over Vandenbeusch.
Vandenbeusch is 42 and had been working in the area, where the Nigerian Islamist Boko Haram group has operated in the past, since September 2011.
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French President Francois Hollande said Paris was doing "everything possible" to find and free him, but warned French citizens against putting themselves in harm's way.
"I would... Ask all of my compatriots who live or travel in what I would qualify as high-risk areas to do nothing to put their lives in danger or to expose themselves to kidnappings," he said on a trip to Monaco.
"We had expressly advised him not to stay on but he thought he should remain there," French Foreign Minister Laurent Fabius told AFP during a visit to Morocco.
A French diplomatic source, who refused to be identified, warned of an "assassination risk" if his abductors panicked.
A nun who worked with Vandenbeusch told AFP the kidnappers spoke English.
Although Cameroon is predominantly French-speaking, English is spoken in several areas, notably near the Nigerian frontier.
Cameroon government spokesman Issa Tchiroma Bakary said Yaounde feared that the priest had already been taken out of the country, though he did not say where to, and added that the kidnap seemed to bear the hallmark of Boko Haram.