The Filipino boxing champion lost his WBO Welterweight title after his shock defeat to newcomer Horn in the "Battle of Brisbane" on July 2.
Horn, a former schoolteacher, won by the bout in an unanimous decision, prompting calls for a rematch or review.
Few thought Horn could take down the 38-year-old Pacquiao, whose last fight and win was against American Jessie Vargas in November 2016.
The Philippines' Games and Amusement Board (GAB) submitted a formal request to review the fight, which was backed by the losing fighter
. Pacquiao said "as a leader" he had a "moral obligation to uphold sportsmanship, truth and fairness in the eyes of the public."
Of the five judges who re-watched the bout round-by-round, three gave the fight to Horn, one to Pacquiao and another called it a draw.
Even if Pacquiao had been declared the true winner, the WBO said that it didn't have the power to reverse the decision of the three judges, who awarded the fight to Horn, 117-111, 115-113, 115-113.
Horn posted the WBO's decision on his Facebook page
with the message "Just FYI guys," tagging actor
Samuel L. Jackson, retired basketball star Kobe Bryant,
ESPN personality Stephen A. Smith, boxing commentator Teddy Atlas and NFL quarterback Aaron Rogers -- all of who expressed their astonishment that Pacquiao had lost.
The fight had a rematch clause
in the contract, teasing the possibility that the two boxers may meet again, giving Pacquiao a chance for redemption.
The result has been called one of the greatest upsets in the sport's history. In his prime, the Filipino boxer, (59-7-2, 38 KOs), was considered by Ring magazine to be the best pound-for-pound boxer in the world
It sparked a flurry of social media protest from boxing luminaries and fans alike, who perceived a bias towards the Australian fighter in both refereeing calls and the ultimate decision of the judges.
Despite the outcry over the decision, Pacquiao was gracious in defeat.
"I didn't expect (him to be) that tough," he said after the fight. "It's OK. Part of the game. That's the decision of the judges. I respect that."