Boxer Jeff Horn says there's NO DOUBT in his mind he beat Manny Pacquiao -- telling TMZ Sports the WBO's rescoring of his July 2 fight proves he's the real-deal champ.

The Australian fighter just touched down in L.A. and told us he just found out the results of the WBO's review -- in which they had 5 independent judges from around the world rescore the fight without sound. 

The judges scored the fight in favor of Horn -- and he told TMZ Sports the new information should silence his critics once and for all.

"There should be no argument," Horn said ... "It's exactly what I felt on the day and Pacquiao said the same thing after the fight that he thought he'd lost. So the decision stands."

Congrats, champ.