Male-male aggression is widespread in the animal kingdom and subserves many functions related to the acquisition or retention of resources such as shelter, food, and mates. These functions have been studied widely in the context of sexual selection, yet the proximate mechanisms that trigger or strengthen aggression are not well known for many taxa. Various external sensory cues (visual, audio, chemical) acting alone or in combination stimulate the complex behavioral interactions of fighting behaviors. Here we report the discovery of a 10 kDa protein, termed Loligo ?-microseminoprotein (Loligo ?-MSP), that immediately and dramatically changes the behavior of male squid from calm swimming and schooling to extreme fighting, even in the absence of females. Females synthesize Loligo ?-MSP in their reproductive exocrine glands and embed the protein in the outer tunic of egg capsules, which are deposited on the open sea floor. Males are attracted to the eggs visually, but upon touching them and contacting Loligo ?-MSP, they immediately escalate into intense physical fighting with any nearby males. Loligo ?-MSP is a distant member of the chordate ?-microseminoprotein family found in mammalian reproductive secretions, suggesting that this gene family may have taxonomically widespread roles in sexual competition.