Update 1.10pm: The homemade explosive 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.
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, Mr Cadot said.
No-one else was injured and only light damage was incurred.
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 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."
Earlier: A letter bomb has exploded at the French office of the International Monetary Fund, lightly injuring one person, Paris police said.
One employee suffered minor hand and face injuries.
Part of the building was evacuated as a precaution.
No other damage has been reported from the incident in western Paris, a police official said.
It is unclear who sent the letter.
Police experts are at the scene, said the official.
France remains in a state of emergency after a string of deadly Islamic extremist attacks over the past two years.