Background: Prolonged grief disorder (PGD) is characterised by persistent yearning and intense emotional pain, and is often accompanied by identity disruption and social withdrawal. Research has identified various PGD risk factors; however, limited research has examined how relationship to the deceased relates to PGD risk. Aims: This study introduces ‘simpatico’, a novel construct assessing a mourner’s perceived similarity and connection to the deceased as a risk factor for PGD. Grounded in the micro-sociological theory of bereavement, the study hypothesises that simpatico relationships heighten PGD risk because of the particular social deprivations their absence creates. Method: This cross-sectional study was conducted in Turkey via online surveys with 400 bereaved. Data were analysed using exploratory and confirmatory factor analyses, as well as correlation- and regression-based analyses. Results: A nine-item Simpatico Scale was validated within a Turkish bereaved adult sample (N = 400), demonstrating good internal consistency (Cronbach’s α = 0.90). Regression analyses revealed that elevated simpatico scores, particularly felt similarities with the deceased, were significantly associated with PGD symptom severity (β = 0.31, p < 0.001), even when controlling for demographic, cause of death, relationship to deceased and social support variables. Conclusions: Results identify simpatico as a new, particularly influential, interpersonal risk factor for PGD. Further, according to the micro-sociological theory, results suggest that promotion of simpatico relationships among bereaved persons may compensate for the social deprivations resulting from simpatico relationship losses. In these ways, this study identifies mourners at risk and suggests promising ways to intervene to reduce mourners’ risk of PGD.