This study addresses the underlying mechanisms explaining the growing demand for pet fashion. While the literature suggests pet attachment as an antecedent of pet product purchase it is still not clear how this effect relates to pet anthropomorphism, self-expansion through the pet, and attachment in human relationships. To address these gaps, we propose a theoretical model which connects human partner attachment orientation, pet attachment, and the purchase of fashion pet clothing. Pet anthropomorphism provides a process explanation for the effect of pet attachment on purchase intention, and self-expansion through the pet constitutes a contingency factor of this process. To test this framework, a representative online survey of Australian pet owners recruited by commercial online panel provider Kantar-Lightspeed and an experimental study with university students were conducted. The findings contribute to the literature on consumption symbolism, self-identity, and attachment by disentangling and clarifying the influences of pet attachment, anthropomorphism, and self-expansion on the purchase of pet fashion products. Results provide also novel evidence on the impact of human avoidant and anxious partner attachment orientation on pet attachment and its downstream effect on pet product purchase.