The number of Americans without health insurance climbed to 27.5 million in 2018, according to federal data that show the first year-to-year increase in a decade, before the Affordable Care Act began reducing the ranks of the uninsured.

The increase, which follows years of steady declines in the number of uninsured, appears to have been driven by a decline in coverage under public health programs such as Medicaid, according to the report. The number of uninsured rose by nearly two million people overall, the Census Bureau...