The mating behavior of deep-sea squids is shrouded in mystery. The squids for which mating has been observed use a hectocotylus, a modified arm, for the transfer of sperm packets called spermatophores. However, many deep-sea squid species lack a hectocotylus. We present the first in situ observations of mating behavior in a deep-sea squid that has no hectocotylus but instead uses an elongated terminal organ for the transfer of spermatangia, which are released from the spermatophores and burrow deeply into the female tissue. With remotely operated vehicles (ROVs), we observed two mating pairs of the deep-sea squid Pholidoteuthis adami in the Gulf of Mexico. The male adopted a peculiar position during mating, with its ventral side up and its posterior mantle above the female’s head. While the male held the female in what looked like a firm grip, we observed the long terminal organ extending through the funnel of the male, contacting the female dorsal mantle. Examinations of museum specimens show that spermatangia burrow from the outer dorsal mantle into the inner dorsal mantle. This combination of serendipitous in situ observations and archived specimens can be a powerful tool for understanding the behavior of deep-sea animals.