Walmart Stores, the world’s largest retailer and the nation’s largest private employer, said on Tuesday that it would terminate health insurance coverage for about 30,000 part-time workers, joining a string of retailers that have rolled back benefits in response to the Affordable Care Act.

Starting on Jan. 1, Walmart will no longer offer insurance to employees working less than an average of 30 hours a week, a move the retailer said was in response to an unexpected rise in health care costs.

“This year, the expenses were significant and led us to make some tough decisions,” Sally Welborn, Walmart’s senior vice president for global benefits, said in a blog post announcing the changes.

The workers losing their coverage make up about 5 percent of the company’s part-time work force of about 600,000, including in-store, logistics and corporate workers, said Brooke Buchanan, a company spokeswoman. Walmart did not disclose what percentage of the part-time work force would be left without coverage. Many part-time employees were never covered for a variety of reasons.