Anthony Joshua All Smiles With Prince Harry At Charity Event After Loss

Anthony Joshua didn't look like a guy who just lost all his championship belts on Wednesday ... grinning from ear-to-ear with Prince Harry at a charity event!!

The British boxing superstar was back in the gym to support the launch of the Made By Sport campaign, an organization created to help youth connect with sports in disadvantaged communities.

Joshua had a blast with the kids in attendance ... giving out some boxing tips and chatting it up with his buddy, Prince Harry -- an avid boxing fan himself.

The duo have a history of being bros ... and Joshua even once joked that he'd crash Prince Harry and Meghan Markle's wedding to be his best man.

Of course, AJ just lost his WBA, IBF, WBO, and IBO heavyweight titles after Andy Ruiz Jr. pulled off one of the most shocking upsets in boxing history on June 1.

As for Ruiz, he hopped on a private jet and visited the Mexican president on Tuesday ... so everyone's hangin' with power people.