“The majority of inmates in the 18- to 21-year-old cohort are young men of color whom we presume innocent under our laws because they are awaiting trial,” said Bryanne Hamill, one of the board’s strongest voices for eliminating solitary for young inmates. “The evidence showed that solitary confinement will not improve their future behavior, but will reliably convert anger and frustration today into rage and violence tomorrow.”

The plan to further reduce the use of solitary confinement was approved unanimously, 7 to 0, by the city’s Board of Correction on Tuesday morning and is supported by the correction commissioner, Joseph Ponte, who was appointed by Mr. de Blasio last year to spearhead reforms at Rikers. The change would not take effect until January 2016 and is contingent on obtaining financing for additional officers and clinical staff members.

Jail reform advocates who have criticized the Correction Department for years praised the initiative. The New York Civil Liberties Union said the change would “make Rikers a leader in solitary confinement reform.”

“With these reforms, New York City has taken an important stand for basic human rights and reaffirmed its commitment to the safety of prisoners, prison staff and our communities,” said Donna Lieberman, the organization’s director. “An institution as profoundly broken as Rikers Island will require wholesale reform to transform into a humane environment that emphasizes treatment and rehabilitation over punishment and isolation, and these rules are a major step forward.”

Norman Seabrook, president of the 9,000-member correction officers’ union, said the plan, which appeared to take him by surprise, would endanger correction officers, leading to more inmate attacks. He vowed to sue the board for every guard assaulted.

“I’m deeply, deeply bothered by a decision that you would make, jeopardizing the safety and security of inmates and officers,” he told the board on Tuesday.