Background: In the revised ICD-11 and DSM-5-TR, Prolonged Grief Disorder (PGD) has been officially recognized as a distinct mental disorder. This condition is characterized by a grief reaction following the death of a close person that persists beyond the expected period (more than 12 months for adults in DSM-5-TR; more than 6 months in ICD-11), significantly impairing quality of life, social functioning, and psychological stability. To assist in the early detection and diagnosis of PGD, researchers at Weill Cornell Medicine, New York developed the Prolonged Grief Disorder–13, revised (PG-13-R) self-report measure in 2020, based on the diagnostic criteria of ICD-11 and DSM-5-TR. Although this instrument has been translated and validated in several languages, no validated Mongolian version has been available, forming the rationale for this study. Aim: To examine the psychometric properties—including reliability and validity—of the Mongolian version of the Prolonged Grief Disorder-13, revised (PG-13-R) scale. Materials and Methods: Permission to use the PG-13-R was obtained from the original author (H.G. Prigerson) in November 2024. The study proposal and methods were reviewed and approved by the Academic Council of the University of Humanities (Approval No. A/19), adhering to ethical research guidelines. The translation process followed international recommendations, including forward translation, back-translation, and expert review by linguists. A pilot test was conducted to ensure clarity of each item. The main study included 442 participants (81.9% female; mean age = 41.8±12.4 years) who had experienced bereavement at least six months prior to participation. Results: The Mongolian version of the PG-13-R demonstrated excellent internal consistency (Cronbach’s α = 0.92; McDonald’s ω = 0.93). The split-half correlation was r = 0.87, indicating high internal correlation. The test–retest reliability was strong, with an Intraclass Correlation Coefficient (ICC) of 0.82 (item ICCs ranging from 0.55 to 0.88), confirming temporal stability. The Content Validity Index (CVI) was 0.89, indicating good content validity. Regarding criterion and convergent validity, the Mongolian PG-13-R showed statistically significant negative correlations with the WHOQOL- BREF subdomains (physical health, psychological, social relationships, and environment) and moderate positive correlations with HADS anxiety and depression scores, consistent with theoretical expectations. Factor analysis results indicated that the data were suitable for analysis (KMO=0.93; Bartlett’s test p<0.001). A single factor with eigenvalue >1 emerged, explaining 62.1% of the total variance, with factor loadings ranging from 0.63 to 0.87, consistent with the original structure. Conclusion The Mongolian version of the PG-13-R scale exhibited a stable one-factor structure and demonstrated strong reliability and validity. It is a psychometrically sound and effective tool for assessing prolonged grief among the Mongolian population.