HAMILTON, Bermuda—Ariel Holdings Ltd. Chairman and CEO Don Kramer has stepped down, the Bermuda-based reinsurer said Friday.

George Rivaz, who has been president of Ariel Holdings since 2008, has been named CEO.

Mr. Kramer, a Bermuda insurance veteran who founded Ariel in 2005 after raising approximately $1 billion from private equity investors, will become nonexecutive chairman of the company.

Mr. Rivaz will continue in his role as nonexecutive chairman of Ariel Reinsurance Ltd. and Atrium Underwriting Group, the company’s two principal operating subsidiaries, the statement said.

“George was a key player in establishing Ariel Re’s franchise, led our acquisition of Atrium in 2007 and has positioned the company well to take advantage of opportunities ahead,” Mr. Kramer said in a statement.

Privately held Ariel had a market capitalization of $1.7 billion at year-end, having reported record profits for the recently concluded fiscal year.

Mr. Kramer did not immediately respond to a request seeking further comment.