The incident came as a Greek anarchist group claimed responsibility for a failed letter bomb sent to the German Finance Ministry on Wednesday.
After Thursday's explosion in Paris, staff members from the IMF office were evacuated and armed military officers and police guarded the area, in a chic district of western Paris.
It is unclear who sent the homemade explosive, which was like a "big firecracker" and sent by regular mail, said Paris police chief Michel Cadot. He said the IMF office had received threatening phone calls in recent days but they were not necessarily linked to Thursday's incident.
IMF director Christine Lagarde, who is French, said in a statement that she was informed about the explosion while on a trip to Germany. "I condemn this cowardly act of violence and reaffirm the IMF's resolve to continue our work in line with our mandate," she said.
French President Francois Hollande, whose country remains in a state of emergency after 235 people were killed in Islamic extremist attacks over the past two years, told reporters Thursday in southern France: "We are again confronted with an attack. There is no other word for it when you are confronted with a package bomb."
Hollande later said the government would do "everything to find out the origin of this malicious act".
He said the letter bomb bore "a similarity with another event of the same nature in Berlin."
"We are trying to establish the causes of what happened as part of an international investigation," Hollande said.
The secretary who opened the letter was injured by shrapnel in the face and hurt in the eardrum because of a "rather violent noise," the police chief said.
Police searched all four floors of the building, which also houses the World Bank office in France, Cadot said. No one else was injured and only light damages were incurred.
The Greek group that claimed responsibility for the German bombing, Conspiracy Cells of Fire, claimed in an online posting Thursday on a Greek left-wing website that the attack was part of a concerted campaign by international anarchist groups.
Many Greeks resent austerity measures imposed by IMF and the European Union linked to Greece's bailout program.
The bomb sent to Germany, containing low-grade explosives like the ones used in fire crackers, was destroyed Wednesday before it could explode.