“The fact is: No matter how much we might work to build ‘IDJ’ as a brand, that brand could never be as powerful as each of IDJ’s constituent parts,” Lucian Grainge, the chairman of Universal, wrote in a note to employees on Tuesday.

The changes to Island Def Jam also confirmed what has been whispered about among music executives for months: that Mr. Weiss — a highly regarded executive who came to Universal as part of a swap that sent Universal’s former chairman, Doug Morris, to Sony — had never found a clear role at Universal. He had been hired to run the label’s New York labels, which include Republic and Island Def Jam, but that job was ill-defined, and the labels remained somewhat autonomous.

Universal’s announcement on Tuesday said that Mr. Weiss had stepped down from his position “in order to enter into discussions with U.M.G. about a new venture together.” But that venture is unclear, and many in the music industry expect that Mr. Weiss may leave the company.

“Barry is an exceptional music executive, and I’m enormously grateful for all of his invaluable contributions during a time of transition for our East Coast companies,” Mr. Grainge said in the note to employees. “He has positioned Def Jam, Island, Motown and Republic to where they are today — poised for even greater levels of success — and we’d love to keep him in the family.”

As part of the reorganization, Steve Bartels, who had been president and chief operating officer of Def Jam, will become that label’s chief executive. Island will continue under its president, David Massey. And Motown Records, which had been under the Island Def Jam umbrella for the last several years after being part of Republic, will relocate to Los Angeles and become part of the Capitol Music Group. Ethiopia Habtemariam, who had been executive vice president of Motown, will become president.