The blog entry, on an Angels fan site, showed a screen shot taken from the FOX broadcast of Monday's Game 3, won by the Angels, 5-4, in 11 innings. Replays show Rivera looking down at the baseball before spitting, and then the camera cuts away.
The Commissioner's Office reviewed video from the FOX broadcast, as well as still photographs from the game.
"We looked at the video and still photography and there was no evidence that Rivera was spitting on the ball," said Pat Courtney, Major League Baseball's vice president of public relations.
Rivera -- famed for authoring his illustrious career on the power of one pitch, a devastating cutter -- entered Game 3 in the 10th inning with a runner on second and no outs, working out of a bases-loaded, one-out jam to move the game to the 11th.
Yankees manager Joe Girardi said that he heard about the video from one of the coaches on the team bus traveling to Angel Stadium on Tuesday, and then called general manager Brian Cashman to find out what the fuss was about.
"I kind of laughed," Girardi said. "Mo's been throwing one pitch for a long time. ... The one thing about a spitter is it consistently does not go one way like Mo's ball consistently goes one way. So I kind of laughed at it.
"MLB has investigated, they have nothing about it. We just move on it. To me, it's a dead story. I caught Mo for four years, and I know for sure he never did anything."
Yankees pitching coach Dave Eiland told the New York Post that Rivera was "absolutely not" guilty of such an infraction.
"I would put my life on it that he didn't do that stuff," Eiland told the newspaper. "He's been pitching for years and not one word has ever been mentioned."
Angels manager Mike Scioscia said he would have been surprised if Rivera had intended to gain an edge.
"This is the first I'm hearing about this," Scioscia said. "I didn't even know that there was any indication that it's been looked at. Never. There are certainly some guys that might be suspect -- never Mariano with anything that I've heard or been part of. And I'd be shocked if there was anything to that."
Bryan Hoch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.