WASHINGTON—President Trump on Sunday said he discussed withdrawing U.S. troops from Syria with Turkish President Recep Tayyip Erdogan and made clear he is sticking by the decision despite criticism from lawmakers and military leaders.

Mr. Trump said on Twitter it was a “long and productive call” and they discussed “ISIS, our mutual involvement in Syria, & the slow & highly coordinated pullout of U.S. troops from the area. After many years they are coming home. We also discussed heavily expanded Trade.” The call took...