Mayor Martin J. Walsh said on Sunday that he had signed a measure banning single-use plastic bags in Boston, despite worries the city’s low-income residents and seniors would bear the financial brunt of the new rules. “My concern about [banning] plastic bags is just the cost,’’ Walsh said. “I just think the cost is going to be shifted over to the consumer.’’

Boston joins 59 other municipalities statewide and hundreds across the nation, including Seattle and Washington, D.C., in adopting aban.

It takes effect next December, giving stores and shoppers time to prepare.