Dosidicus gigas (the Humboldt squid) is a widely distributed and ecologically important predator in the eastern Pacific Ocean, but its mating behaviour is poorly understood. Individuals of this species have undergone a drastic change in size at maturity in the last years. We investigated mating activity of Humboldt squid in the Gulf of California in 2013, 2014, and 2015 by quantifying spermatangia deposited in the tissue of the buccal area. In 2015, we encountered the smallest mean mantle length of mature specimens recorded to date in the Gulf of California. In all years, numerous males were encountered that had been mated by other males. Spermatangia in males were deposited on the tissue in similar numbers and in the same location as normally occurs in females (the buccal area), suggesting that male-to-male mating behaviour is similar to male-to-female. This behaviour is referred to as same-sex sexual behaviour and has been described for various taxa, including other cephalopods. Overall similarity in mating frequency between males and females and in body size of mated individuals (in 2015) suggests non-discriminative and brief encounters with body size being a cue for mating. This mating strategy may be beneficial for males, as Humboldt squid live in groups where competition for mates is likely high. The energetic costs of male-to-male mating events may be counterbalanced by the fitness profits of indiscriminate mating behaviour.