The comment came after France's Interior Minister Manuel Valls predicted more arrests of Islamic extremists and warned of possibly hundreds of home-grown militants.
"In a large number of prisons, there is no Muslim chaplain," said Abdelhak Eddouk, a chaplain at Fleury-Merogis detention centre in the Paris region.
"Where there is one, radicalism does not take on the same dimensions. Even if the chaplain does not prevent the actions of radical detainees, his presence can lessen their influence," he said.
He said there were around 160 chaplains in the country but around three times that number was needed, adding that the 13 new ones budgeted for next year was far from sufficient.
Interior Minister Valls said at the weekend that France's prisons needed "trained imams" and not "self-proclaimed imams".
Eddouk said that the self-styled imams in prisons had no proper religious training but had picked up whatever knowledge they had through the Internet, books or talks with other individuals in jail.
"They see it as normal to proclaim themselves imams," he said, noting that a properly trained imam had at least four years of studies.
A nationwide police sweep at the weekend left one man dead and 12 others in detention on suspicion of either being involved in the bombing of a Jewish grocery last month or of planning other anti-Semitic attacks.
The dead man, whose DNA were found on the grenade that hit the store, is believed to have converted and been radicalised during a prison term for drug dealing.