Prolonged grief disorder (PGD), a new diagnosis in some classification systems, has gained prominence due to the high mortality rates observed during the Covid-19 pandemic. Herein, the prevalence of PGD (diagnosed with structured clinical interviews), death-related features, and associated clinical factors were investigated among outpatient psychiatric patients who lost a first-degree relative within the past 12–24 months. PGD was diagnosed in 30/68 patients (44.1%). PGD development did not differ based on cause of death (Covid-19-related vs. other causes) but was associated with higher age of the bereaved, younger age of the deceased, and degree of kinship. Higher rates of depression, insomnia, and anxious attachment were also observed in PGD patients. Finally, the unexpectedness of death predicted the development of PGD. Due to the high prevalence of PGD among psychiatric patients, clinicians should be aware of the disorder, monitor grief processes in high-risk patients, and consider PGD in treatment planning.