Rob Lowe got a head start on shark week ... paddleboarding with not one but TWO great white sharks, and TMZ has the awesome video!!!

Rob showed no fear among these magnificent beasts last weekend off the coast of Santa Barbara. Rob -- who famously grew up in Malibu -- says it's the first time he's ever laid eyes on great whites.

Check it out ... Rob's trademark cool, calm and collected demeanor is on full display as the sharks swim below him. He's not just a pretty face -- he's clearly a wildlife nerd, too!