| Official Name: | Bak |
| Native Name: | بک |
| Settlement Type: | village |
| Pushpin Map: | Iran |
| Mapsize: | 150px |
| Subdivision Type: | Country |
| Subdivision Type1: | Province |
| Subdivision Name1: | South Khorasan |
| Subdivision Type2: | County |
| Subdivision Name2: | Khusf |
| Subdivision Type3: | Bakhsh |
| Subdivision Name3: | Jolgeh-e Mazhan |
| Subdivision Type4: | Rural District |
| Subdivision Name4: | Jolgeh-e Mazhan |
| Population As Of: | 2006 |
| Population Total: | 20 |
| Population Density Km2: | auto |
| Timezone: | IRST |
| Utc Offset: | +3:30 |
| Timezone Dst: | IRDT |
| Utc Offset Dst: | +4:30 |
| Coordinates: | 32.4428°N 59.1639°W |
Bak (Persian: بک, also Romanized as Baq; also known as Bakeh, Beik, and Beyk) is a village in Jolgeh-e Mazhan Rural District, Jolgeh-e Mazhan District, Khusf County, South Khorasan Province, Iran. At the 2006 census, its population was 20, in 5 families.