Its latest report cited a worsening spillover from China's economic slowdown as well as the impact of low oil prices on emerging markets such as Brazil.
It also highlighted persistent economic weakness in Japan, Europe and the United States.
The gloomier picture sets the stage for the IMF and the World Bank to call this week for more co-ordinated global action to support growth.
"In brief, lower growth means less room for error," IMF's chief economist, Maurice Obstfeld, told a news conference, adding that "scarring effects" from years of tepid growth could in turn weaken demand, thin the workforce and reduce potential output further, creating a scenario of "secular stagnation".
In its report, the IMF warned that the rise of nationalist parties in Europe, the June 23 'Brexit' referendum, and anti-trade rhetoric in the US presidential campaign posed threats to the global economic outlook.