Sex estimation from skeletal remains, particularly when the pelvis is not preserved, has long been one of the major challenges in biological anthropology. The present study was conducted to examine sexual dimorphism in cranial and mandibular traits and to develop population-specific criteria for sex determination in the Parthian population of the Liyarsangbon site. For this purpose, seventy-seven adult human skeletons curated at the Biological Anthropology Laboratory of the University of Guilan were analyzed. Biological sex was first determined using well-established pelvic indicators and was subsequently used as a reliable reference. A series of quantitative cranial and mandibular landmarks were then measured, and qualitative cranial–mandibular traits were assessed using ordinal scoring systems. Statistical analyses included independent samples t-tests for metric variables and Mann–Whitney U tests for ordinal traits. The findings indicate that several cranial and mandibular variables, including maximum cranial length, selected dimensions of the cranial base, mastoid length, and ramus-related measurements of the mandible, exhibit significant sexual dimorphism. In contrast, a number of traits—particularly palatal measurements and certain qualitative features such as mental eminence—did not show statistically significant differences between males and females. The coexistence of traits that conform to global sex estimation standards alongside others that diverge from them suggests that sexual dimorphism in the Liyarsangbon population is not uniform. Some cranial and mandibular traits appear to be population-specific and may reflect local genetic, environmental, or biocultural influences. These results underscore the importance of developing localized sex estimation standards and demonstrate that uncritical application of global criteria may reduce diagnostic accuracy. Population-based approaches therefore provide a more reliable framework for sex determination in Parthian-period skeletal assemblages from northern Iran.