In a sample of 229 individuals who recently undertook a large-scale sacrifice by relocating for their romantic partner’s job or schooling (i.e., accompanying partners), we tested preregistered predictions linking accompanying partners’ attachment insecurities (i.e., attachment anxiety and attachment avoidance) and relationship functioning (i.e., relationship quality, relationship conflict, and move-related relationship benefits). We also examined whether any negative associations found among accompanying partners’ attachment insecurities and relationship functioning could be buffered by partner prosociality in the form of perceived partner gratitude (i.e., partners’ expressions of move-related and general gratitude) and perceived partner sacrifice (i.e., partners’ daily sacrifice behaviors and general willingness to sacrifice). Results showed that more insecurely attached accompanying partners reported worse relationship functioning after moving than their secure counterparts. Although gratitude and sacrifice did not buffer insecurely attached individuals’ relationship conflict, both perceived partner general gratitude and willingness to sacrifice partially buffered avoidantly attached individuals from experiencing lower relationship quality, while move-related gratitude helped avoidantly attached individuals to feel that the move benefitted their relationship. Meanwhile, perceived partner sacrifice behaviors buffered anxiously attached individuals from experiencing lower relationship quality. This is the first study to demonstrate, in an ecologically valid sample, the implications of a large-scale sacrifice for insecurely attached accompanying partners’ relationship functioning, as well as the protective effects of perceiving a partner’s prosociality following the major life transition of job relocation.