Mayor Bill de Blasio, opening a new front in his quest to expand workers’ rights, said on Tuesday that he would offer six weeks of fully paid parental leave to 20,000 public employees, making New York City among the most generous municipal family-leave providers in the nation.

New mothers and fathers, including workers who adopt a child or accept one into foster care, would be eligible for the benefits, which far outstrip those currently available to most federal and New York State employees. The mayor plans to enact the plan by executive order, and the benefits would go into effect next year.

Mr. de Blasio, a Democrat who is one of the country’s most liberal urban leaders, has made labor rights a hallmark of his tenure, mandating paid time off for sick workers and urging higher wages. His announcement comes as some Fortune 500 companies, including Microsoft and Netflix, have moved to expand benefits and time off for new parents.

“Too many new parents face an impossible choice: taking care of their child or getting their paycheck,” Mr. de Blasio said in a statement. “This is a common-sense policy that will make for healthier and more financially stable working families.”