Landlords of upscale properties across the U.S. are bracing for rough conditions in 2017 that will likely force them to slash rents and offer deep concessions as a glut of supply brings a seven-year luxury-apartment boom to an end.

The turnaround follows a more-than-26% jump in U.S. apartment rents since early 2010, far outstripping inflation and income growth. But in 2016, rents rose a modest 3.8%, a significant drop from the recent high of 5.6% year-to-year growth in the third quarter of 2015, according to a report to be...