A Staten Island judge on Tuesday dealt a major setback to Mayor Zohran Mamdani’s push to raise taxes on the wealthy, saying that the city had mishandled the rollout of a tax on high-end second homes and must start over. The judge, Wayne M. Ozzi of State Supreme Court on Staten Island, sided with a group of homeowners who had sued the city. The homeowners had argued that the Mamdani administration did not try hard enough to determine who should owe the tax before moving toward collecting it.

“Homeowners are being substantially harmed and penalized needlessly” by the city’s “method of implementing the tax law,” Justice Ozzi wrote. The lawsuit did not challenge the legality of the tax on second homes, also known as pieds-à-terre. Unveiled by Gov. Kathy Hochul in the spring and backed by the mayor, the tax was popular among voters and hailed by Mr. Mamdani’s supporters as a way to make good on his pledge to “tax the rich.” It was expected to close a budget gap by raising some $500 million annually to fund city services.

But the bumpy rollout of the policy on a short timeline drew immediate criticism from the mayor’s opponents, and some allies, including Ms. Hochul. “City Hall botched this rollout and should have just admitted the errors and fixed its own mistake, instead of wasting time and taxpayer dollars by fighting it in court,” said Randy Mastro, a lawyer for the homeowners and a former city official who has criticized the mayor.

Justice Ozzi ordered the city to take down its online list of nearly one million properties and replace it with a more limited version, cancel mailed notices, and send new ones with detailed information. The city is expected to seek a stay of the decision. The first tax bills typically go out in November, and funds must be collected by spring.