WASHINGTON—Casino mogul Steve Wynn is stepping down from his post as Republican National Committee finance chairman following a Wall Street Journal report alleging a decadeslong pattern of sexual misconduct by him.

“Effective today I am resigning as finance chairman of the RNC,” Mr. Wynn said in a statement Saturday.

“The unbelievable success we have achieved must continue,” he added. “I thank the president for the opportunity to serve and wish him continued success.”

RNC Chairwoman Ronna Romney McDaniel confirmed that she had accepted Mr. Wynn’s resignation earlier Saturday.

The Journal on Friday reported that shortly after Mr. Wynn opened his flagship Wynn Las Vegas in 2005, a manicurist who worked there told a colleague Mr. Wynn forced her to have sex. The casino billionaire later paid the manicurist a $7.5 million settlement, the Journal reported.