The IMF made the unusually frank admission in an evaluation of how it handled Greece's debt crisis, which triggered financial turmoil across the eurozone.
The Greek economy has been kept afloat for the past three years by rescue loans from Europe and the IMF in exchange for harsh austerity measures that have worsened the recession, currently in its sixth year. It has so far received about 200 billion euros (£170 billion) in loans from a rescue programme totalling 240 billion euros (£204 billion).
While the measures have reduced Greece's budget deficit, they have left the country mired in a much deeper recession than the IMF and its European partners in the bailout forecast three years ago. Unemployment is 27%.
The report said the IMF and its partners in the bailout had significantly underestimated how much austerity measures, such as spending cuts and tax increases, would affect the economy. However, the fund insisted that it could not have slowed the pace of belt-tightening.
The IMF said there were notable successes in the bailout as well. Greece remained in the eurozone and the spillover effect on the global economy was relatively contained. "However, there were also notable failures," it said.
It described the first two years of the rescue programme as a "holding operation" while the other 16 European Union countries that use the euro currency grappled with the enormity of the unfolding crisis and built "firewalls" to try to stave off a contagion effect. And the report said rescue lenders had allowed Greek debt to remain too high until private bonds were eventually restructured last year.
Some of the failure was blamed on Greece itself, with the IMF saying it had stalled on structural reforms of its economy such as privatisations and improving tax collection. The document said the lending agency and its partners had been too optimistic about the political conditions for pushing forward with such reforms.
Greece restructured its privately-held bonds last year. But the country is still struggling to stabilise its national debt - which is set to rise to 175% of its annual output this year. The country has been promised additional relief from the eurozone if Athens delivers its promise to balance its budget this year. What form that relief will take remains unclear and speculation has increased that a further debt restructuring is on the cards.