The ordinance calls for city water to be fluoridated by March 2014.
Opponents also say council members rushed into action without a public vote. They plan to collect signatures to force a referendum in May 2014.
Voters in Portland twice rejected fluoridation before approving it in 1978. But that plan was overturned before any fluoride was ever added to the water.
Portland's drinking water already contains naturally occurring fluoride, though not at levels considered to be effective at fighting cavities.
Portland City Commissioner Nick Fish, who co-sponsored the plan, has said more than 200 million Americans drink water with added fluoride, and it doesn't appear to have caused great harm. Most mainstream health organizations, such as the American Medical Association and American Dental Association, endorse it as safe.
Public fluoridation came up this week in Phoenix when a public stir prompted re-examination of a policy in place since 1989. After a contentious hearing Tuesday, council members voted to continue adding fluoride to the water in the nation's sixth-largest city.