SANTIAGO, Chile — Chile will become the first nation in the Americas to ban retail businesses from using plastic bags, an initiative aimed at protecting the country’s 4,000-mile coastline.

The measure, which Congress approved unanimously this week, gives large retailers and supermarkets six months to comply.

Small and medium-size businesses, including neighborhood shops, will have two years to abide by the new rules. In the meantime, they may hand out up to two plastic bags per client.

The measure is President Sebastián Piñera’s first legislative victory since taking office in March. Mr. Piñera expanded the scope of a bill introduced last year by his predecessor, Michelle Bachelet, that sought to prohibit the use of plastic bags in more than 100 coastal towns.