Millions of people are mourning the death of a loved to COVID-19. According to previous studies, the circumstances of coronavirus disease-related deaths may lead to dysfunctional grief. The purpose of this study was to introduce the Polish adaptation of the Pandemic Grief Scale (PGS) as well as to assess the relationship between dysfunctional grief due to a COVID-19 death, resilience and perceived social support. The adaptation was carried out on a general population sample of 286 individuals aged 18–54 years, with the evaluation being performed on a group comprising 214 people aged 18–78 years, who lost a loved one during the pandemic. The Polish version of PGS revealed a single-factor structure with strong internal consistency (α = 0.89). The PGS scores were associated with measures of complicated grief (Inventory of Complicated Grief), depression (Kutcher Adolescent Depression Scale) and lower resilience (Resilience Scale 14), which confirmed the scale’s convergent validity. No relation between PGS scores and health behaviors (Inventory of Health Behaviors) was observed, which confirmed the scale’s discriminant validity. The results of the bootstrapping technique revealed that resilience mediates the relationship between perceived social support (Multidimensional Scale of Perceived Social Support) and dysfunctional grief (total mediation). The results of this study suggest the need for practitioners to focus on resilience-enhancing interventions and perceived social support in order to improve mental health in people who lost their loved ones during the new coronavirus pandemic.