WASHINGTON—The global economy has almost certainly entered a recession affecting most of the world, with a severity unmatched by anything aside from the Great Depression, the International Monetary Fund said Tuesday.

The IMF, in a new outlook, said the world economy is expected to contract by 3% in 2020 as the coronavirus pandemic causes nations around the world to close down, compared with a contraction of 0.1% in 2009, the worst year of the previous recession. This year’s decline amounts to about $2.7 trillion of global...