The gold medalist detailed the cruel act to his half-million Instagram followers from Belgium on Wednesday ... showing the passenger side window of the team's van completely shattered, with law enforcement on scene to check things out.

"Crazy start to the trip," Martin said.

Martin was relieved their bikes weren't in the vans at the time of the break-in ... 'cause those are pretty important.

There was some more luck, too ... Martin -- who won it all in the Men's BMX freestyle at the 2020 Olympics -- and his teammates recovered some items tossed at a nearby park thanks to some bag-tracking gadgets.

However, Martin said his wallet and backpack were nowhere to be found among the ditched stuff. To make matters worse, a fellow team member discovered his "choccies" were eaten at some point in the heist.

"It’s unfortunate when people feel the need to do things like this," Martin said of the experience.