When you need a calm, soothing voice to get you through a traumatic event, you don't think of Mike Tyson ... but that's exactly what happened to a motorcycle crash victim.

Ryan Chesley tells TMZ ... he was on his way to Las Vegas this week when a cab cut him off on the freeway ... sending him flying off his chopper.

Chesley says Tyson saw the whole thing ... slammed on his brakes and stopped traffic. Chesley says Tyson -- mindful the victim might be in shock -- kept him calm until paramedics arrived.