Eric Adams, N.Y.C.’s likely next mayor, wants to turn shuttered hotels into permanent housing for the homeless.

Hotels in New York City that have been left empty by the pandemic would be converted into “supportive housing” that provides a wide range of assistance to people struggling with mental illness or substance abuse and to people leaving the prison system, under a plan proposed on Monday by Eric Adams, who is likely to be the city’s next mayor.

More than 20 percent of the city’s hotels are now closed, a trade association says. At the same time, the city faces a homelessness crisis, growing sentiment against warehousing homeless people in barrackslike shelters and a lot of severely mentally ill people living in the streets.

“The combination of Covid-19, the economic downturn and the problems we’re having with housing is presenting us with a once-in-a-lifetime opportunity,” Mr. Adams, who won the Democratic primary for mayor in June, said as he stood outside a boarded-up hotel in Sunset Park, Brooklyn. “Use these hotels not to be an eyesore, but a place where people can lay their eyes on good, affordable, quality housing.”

Details of the plan were thin. Mr. Adams mentioned the possibility of 25,000 converted hotel rooms, but he said that he would focus on boroughs outside Manhattan, where the number of rooms in closed hotels is much smaller than that.

He was not clear about whether there was any overlap between his plan and those that the current mayor, Bill de Blasio, and the former governor of New York, Andrew M. Cuomo, have already launched to build 25,000 supportive housing units in the city by about 2030. A spokesman for Mr. Adams’s campaign said that Mr. Adams was also considering converting rooms in former hotels that have already become homeless shelters into permanent supportive-housing apartments, something that Mr. de Blasio has also discussed.

The nexus of hotels and homelessness has been a contested one during the pandemic. Early in the lockdown imposed to stem the spread of the coronavirus, thousands of people who had been living in dorm-style shelters were moved to hotel rooms, mostly in Manhattan where their presence led to complaints from some residents about harassment and sometimes violence. The city has since moved most of those people back to group shelters.

Several advocates for homeless people and for supportive housing endorsed Mr. Adams’s plan and stood with him at the news conference. “Adams can be the mayor who uses this inflection moment to change the trajectory on homelessness,” Laura Mascuch, executive director of the Supportive Housing Network of New York, said in an interview. “We look forward to working with Adams to implement the strongest supportive housing program in the nation.”