Socio-sexual and mating behaviors, to our knowledge, have not been previously documented among free-ranging beluga whales, Delphinapterus leucas, but they have been described in detail for captive belugas. We report on the fi rst photodocumented interaction and display of socio-sexual and apparent mating behavior of noncaptive beluga whales in Cook Inlet, Alaska. This behavior was seen on two different days in the same river mouth in uncharacteristically clear waters of upper Cook Inlet. On 24 April 2014, social and possible mating behaviors were observed and photographed for approximately 12 min within a group of nine adult beluga whales in the mouth of Middle River on the west central side of Cook Inlet. A total of 136 photographs were taken at a radial distance > 500 m. On 7 May 2014, similar behaviors were observed among four adult beluga whales in the same location for about 7 min. The second group was not photo-documented due to flight limitations. In both circumstances, affiliative behavioral events such as echelon and contact swimming, and socio-sexual behaviors such as ventrum-to-ventrum contact, ventral presentations, pelvic thrusting, nodding, and rubbing were observed. These behaviors resemble those previously reported for captive beluga mating behaviors and copulation. Similarities between these observations with captive mating behaviors, and the timing of ovulation and peak calving periods from other wild beluga populations, provide strong evidence that mating occurs during early spring months in Cook Inlet.