Offspring-parent attachment bonds promote a proximity seeking – exploration balance in young, facilitating resilience when faced with novel experiences and environments. Research has suggested that, even in adulthood, pet domestic dogs (Canis familiaris) often form attachment bonds towards humans that resemble offspring-parent relationships. However it is unknown if dogs form the same kind of attachment bonds to other species in adulthood, including cohabitant dogs, or if the dog-human relationship is unique in this respect. In the current study we used the Secure Base Test to classify dog-human and dog-dog dyads into attachment style categories. We also evaluated behavioral indicators of stress reduction, proximity seeking and exploration. As in prior studies, we found that the majority of our dog-human dyads met the criteria for offspring-parent type attachment relationships; with individual differences in attachment style influencing the degree of stress reduction and exploration observed. Conversely, 41 out of the 43 dogs evaluated did not meet the traditional criterion for attachment towards their cohabitant dog. Instead behaviors observed in dog-dog relationships better matched those described in sibling attachment research. Overall, companion dogs were significantly less likely to elicit behaviors associated with attachment security in a focal dog when compared to owner presence. Dog-human attachment may play a distinct and important role in the success and resilience of adult dogs living in at least some anthropogenic environments. Bonds formed with other adult dogs may still be important, but likely serve a different function.