Employees were evacuated from the building near the Arc de Triomphe in the heart of the capital after the late morning explosion which was caused by a firework.
IMF chief Christine Lagarde condemned the incident as a "cowardly act of violence" and insisted the fund would continue its work "in line with our mandate," a statement from her office said.
"We are working closely with the French authorities to investigate this incident and ensure the safety of our staff."
Since early 2015, France has been hit by a series of deadly attacks by Islamic extremists.
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The secretary, who works for an unnamed senior manager, suffered facial injuries caused by "shards, as well as a blast to the eardrums" but is out of danger, Paris police chief Michel Cadot said.
"It appears to have been a firework or a big firecracker," he said, describing the device as "relatively rudimentary and nothing like a bomb".
Armed police and soldiers earlier secured the area around the IMF offices and anti-terrorism prosecutors opened an inquiry into the incident.
The incident occurred more than six years after the group waged a similar campaign targeting European officials.
A Greek police source in Athens said "it is rather soon" to link the two incidents, adding: "We are cautious (and) remain vigilant."
He noted that the letter sent to the IMF was destroyed in the blast, while the Berlin parcel bomb was made safe, with the sender's return address clearly marked on it.