"I didn't realize it was coming now," said one Sunday shopper at a local supermarket.
Others remembered, but they still weren't happy.
"I think its kind of a pain in the butt, actually," said another.
Store managers said they expect that a few customers might be unhappy, but all understand it's better for the environment to get rid of the ubiquitous plastic bags.
To ease the pain, some stores like Safeway and Whole Foods gave away reusable bags on the first day of the big change.
Other shoppers, however, have been using their own bags for years and didn't even notice the change.
Those who don't want to bring their own reusable bag can pay a nickel for a paper bag, under the new rule.
The City Council unanimously approved the plastic bag ban last December, joining other cities across the country. Nearby communities such as Bainbridge Island, Mukilteo, Edmonds, Bellingham and Portland, Ore., also have banned plastic bags.
Plastic bags alone make up 1 percent of the trash in Seattle and have long been blamed for littering streets, fouling oceans and harming marine life.
Before the ban, Seattleites carried off about 292 million single-use plastic bags and 68 million paper bags each year.