Employees were evacuated from the building near the Arc de Triomphe monument in the heart of the capital “as a precaution” after the late morning explosion, a police source said.
IMF chief Christine Lagarde condemned it as a “cowardly act of violence”.
She reaffirmed “the IMF's resolve to continue our work in line with our mandate”, according to a statement issued by her office.
“We are working closely with the French authorities to investigate this incident and ensure the safety of our staff.”
French President Francois Hollande called it an “attack”, saying it showed that “we are still targeted.”
The victim, a secretary for an unnamed senior manager, suffered light burns from the blast which might have been caused by a firework, a police source told AFP, asking not to be named.
Armed police and soldiers were seen securing the area around the IMF offices.
France has been the victim of a series of deadly attacks by Islamic extremists since 2015.
Although no link has been established at this stage, a Greek far-left group claimed Thursday that it had sent a parcel bomb to the German finance ministry in Berlin, more than six years after waging a similar campaign targeting European officials.
Claim by 'Conspiracy of Fire'
The Conspiracy of Fire Nuclei group admitted responsibility in a statement posted on an anti-establishment website, but only mentioned Finance Minister Wolfgang Schaeuble's office as a target.
German police said they had discovered the “explosive” package at Schaeuble's office on Wednesday, a day before he was due to host his new US counterpart Steven Mnuchin.
Conspiracy of Fire Nuclei, listed as a terrorist organisation by the United States, in 2010 sent letter bombs to foreign embassies in Greece and to three European leaders — then European Commission chairman Jose Manuel Barroso, German Chancellor Angela Merkel and then Italian prime minister Silvio Berlusconi.
A Greek police source said the claim was “most probably” authentic.
Germany and the IMF are blamed by many Greeks for imposing years of public sector cuts and reforms in exchange for bailout packages needed to prop up the recession-hit country.
Greece's interior ministry said authorities in both Greece and Germany were working together on the case.
A Greek police source said the package had a Greek stamp. The “sender” of the parcel was given as a deputy leader of the opposition New Democracy party — along with his real address.