President-elect Donald Trump is expected to nominate Exxon Mobil Corp. Chief Executive Rex Tillerson to be secretary of state, a transition official said Saturday, a selection that would reach outside the traditional foreign policy establishment to elevate a global business deal-maker.

Mr. Trump hasn't yet made a final decision, the official said, but the president-elect heaped praise on Mr. Tillerson in an interview released Saturday.

“He’s...