CLEVELAND — Tim Tebow, the Heisman Trophy winner, reality television star and former N.F.L. player who was supposed to be one of the biggest names to appear at the Republican National Convention, said Thursday that he would not speak after all.

In a video he posted to Instagram, Mr. Tebow called reports that he would be a speaker “a rumor.”

“It’s amazing how fast rumors fly,” he said. “And that’s exactly what it is. A rumor.”

He cited his commitment to the Tim Tebow Foundation, a children’s philanthropy, as taking priority over politics.