Bam Margera took a serious tumble while showing off his skateboarding skills -- and we've got the clip of the exact moment it all went down.
Check it out ... the pro skateboarder whizzes back and forth on the ramps, when suddenly, on one descent, his left leg lags behind ... sending him hurtling towards the ground.
As you can see, the former "Jackass" star's clearly in agony, letting out a loud "F***!" while writhing in pain on the ground. His buddy finds himself in a tough spot here ... watching helplessly, unsure of what to do next or how serious Bam's pain really is.
Turns out, it was quite bad ... 'cause he headed straight to the hospital after the wipeout. But, he luckily managed to avoid surgery and only needed a knee brace and some crutches instead.
The skateboarder was also comforted by his fiancée Dannii Marie ... who was by his side at the hospital. Looks like their love is still rolling strong!
Bam kept his fans in the loop ... taking to IG to share the news of his MCL tear -- which is essentially a stretch, partial tear, or complete tear of the medial collateral ligament.
Hopefully, he'll kick his recovery into high gear and be back on his feet soon. Feel better!