| Official Name: | Tärnsjö |
| Pushpin Map: | Sweden Uppsala#Sweden |
| Subdivision Type: | Country |
| Subdivision Name: | Sweden |
| Subdivision Type3: | Municipality |
| Subdivision Name3: | Heby Municipality |
| Subdivision Type2: | County |
| Subdivision Name2: | Uppsala County |
| Subdivision Type1: | Province |
| Subdivision Name1: | Uppland |
| Area Footnotes: | [1] |
| Area Total Km2: | 1.89 |
| Population As Of: | 31 December 2020 |
| Population Footnotes: | [2] |
| Population Total: | 1,168 |
| Population Density Km2: | auto |
| Timezone: | CET |
| Utc Offset: | +1 |
| Timezone Dst: | CEST |
| Utc Offset Dst: | +2 |
| Coordinates: | 60.15°N 72°W |
Tärnsjö is a locality situated in Heby Municipality, Uppsala County, Sweden, with 1,221 inhabitants in 2010.