Previously it was thought that giant Pacific octopuses employed a polygynous mating system with only the males mating with more than one female [5]. In this study we did find polygyny with more than one male contributing to more than one female’s egg clutch and also polyandry, females mating with more than one male, or multiple paternity within each female’s egg clutch. Giant Pacific octopuses lay their eggs in strings of connected eggs. Our collection procedure allowed us to determine that multiple paternity is present even within individual strings, as opposed to a string representing fertilization by a single male. Polyandry as a mating strategy is not a surprising tactic given the large numbers of eggs laid, the fact that females are known to store spermatophores after mating, and given that octopuses are terminal phase spawners allowing just one chance to produce viable offspring [4,34]. Thus females that allow more than one male’s sperm to sire their progeny may increase the chance that at least one of the pairings will produce viable offspring. Finally multiple paternity is not unique within cephalopods has been documented in other octopuses and squid