The lawsuit, filed in US District Court in Boston, seeks nationwide class-action status.
Wal-Mart, the largest private US employer, began offering health insurance benefits to same-sex spouses last year, after the US Supreme Court in 2013 struck down part of the Defense of Marriage Act that denied federal benefits to married gay couples. Even after that change, the lawsuit says, Wal-Mart workers still live with the uncertainty of losing spousal coverage.
"Benefits provided by Wal-Mart as a matter of grace ... are not secure and could potentially be withdrawn just when large health care costs are incurred," the lawsuit says.
Jackie Cote, who has worked at Walmart stores in Maine and Massachusetts since 1999, said in the lawsuit that her wife, Diana Smithson, developed cancer in 2012 and the denial of insurance led to more than $150,000 in medical debt.