Portland, Ore., which never fluoridated its water supply and over time earned the distinction as the biggest city in the country to just say no, reversed course on Wednesday with a unanimous vote by the City Council to add fluoride beginning in early 2014.

The decision, which will cost the city about $5 million to carry out, was seen by both supporters and opponents as fraught with significance. Many Portlanders treasure their city’s quirky distinctiveness. Others said its leadership role as the largest city in a state that is mostly nonfluoridated — and has some of the worst tooth-decay problems in the nation, according to various medical studies — made the new course long overdue.

The city’s water system serves about 900,000 people, or almost one-fourth of Oregon’s population, including some in communities outside the city limits.

“It isn’t just time for Portland to enter the 21st century — we have some business to make up from the 20th century,” said Randy Leonard, the public safety commissioner, who was interrupted several times by shouts from the audience. “This is not an issue for the faint of heart.”