| Official Name: | Posht Mesar |
| Native Name: | پشتمسار |
| Settlement Type: | village |
| Pushpin Map: | Iran |
| Mapsize: | 150px |
| Subdivision Type: | Country |
| Subdivision Type1: | Province |
| Subdivision Name1: | Gilan |
| Subdivision Type2: | County |
| Subdivision Name2: | Sowme'eh Sara |
| Subdivision Type3: | Bakhsh |
| Subdivision Name3: | Mirza Kuchek Janghli |
| Subdivision Type4: | Rural District |
| Subdivision Name4: | Markiyeh |
| Population As Of: | 2006 |
| Population Total: | 1,438 |
| Population Density Km2: | auto |
| Timezone: | IRST |
| Utc Offset: | +3:30 |
| Timezone Dst: | IRDT |
| Utc Offset Dst: | +4:30 |
| Coordinates: | 37.3089°N 49.2431°W |
Posht Mesar (Persian: پشتمسار, also Romanized as Posht Mesār; also known as Posht Mīān) is a village in Markiyeh Rural District, Mirza Kuchek Janghli District, Sowme'eh Sara County, Gilan Province, Iran. At the 2006 census, its population was 1,438, in 380 families.