WASHINGTON—Measures taken to curb the spread of the new coronavirus could lower economic activity in the U.S. and other developed countries by a quarter, the Organization for Economic Cooperation and Development said Friday.

In a report made available to leaders of the Group of 20 leading economies for a video conference they held Thursday, OECD economists estimated the likely impact on the sectors most affected by widespread business closures and orders for people to remain at home, and the size of those sectors in each national...