Forget Magnums ... Eli Roth's strapping up with extra small rubbers these days ... thanks to a nasty bite from an OCTOPUS in Hawaii.
Eli tells TMZ, he recently overcame his fear of the ocean while on vacation in the Aloha State -- following a terrifying sea urchin attack in 2010 -- and decided to test the waters with a sea scooter expedition.
But curiosity got the best of him when he reached out to touch a baby octopus ... and the 8-legged bastard bit him on the pinky.
Eli -- the man behind the new Netflix series "Hemlock Grove" -- says his finger bled profusely for hours after the bite, and now he's gotta keep the wound covered with little pinky condoms until it heals.
And the best part, if he ever runs out ... he can always ask Justin Bieber for spares.