"These past weeks, in January, there has been several letters with threats, including one with a revolver bullet," Stephane Sitbon-Gomez told AFP, confirming a Europe 1 radio report.
"We received letters from extreme right, very nationalist people," he said.
"That is the unpleasant hazard of a campaign. We wish neither to publicise this affair nor to dramatise or victimise Eva Joly," said the campaigner for the Europe Ecology-Green Party candidate.
He added that the former French-Norwegian magistrate has been placed under official protection at her request for the past ten days.
The 68-year-old Joly is best known to the French as a campaigning magistrate against corruption who took on some of France's biggest business interests during the 1990s as an investigating judge.
Born Gro Eva Farseth in Oslo, she moved to Paris when she was 20 to work as an au pair.