| Múlaþing | |
| Settlement Type: | Municipality |
| Mapsize: | 250px |
| Pushpin Map: | Iceland |
| Subdivision Type: | Country |
| Subdivision Name: | Iceland |
| Subdivision Type1: | Region |
| Subdivision Name1: | Eastern |
| Subdivision Type2: | Constituency |
| Subdivision Name2: | Northeast |
| Established Title: | Established |
| Established Date: | 2020 |
| Leader Title: | Mayor |
| Leader Name: | Björn Ingimarsson |
| Unit Pref: | Metric |
| Area Total Km2: | 10,671 |
| Population Footnotes: | [1] |
| Population As Of: | 2020 |
| Population Total: | 5,000 |
| Population Density Km2: | 0.47 |
| Postal Code Type: | Postal code(s) |
| Postal Code: | 700-710, 720, 765 |
| Code1 Name: | Municipal number |
| Code1 Info: | 7400 |
Múlaþing (in Icelandic pronounced as /ˈmuːlaˌθiŋk/) is a municipality in eastern Iceland which was formed in 2020 from the merger of Fljótsdalshérað, Seyðisfjörður, Borgarfjarðarhreppur and Djúpavogshreppur.
It is the second largest municipality in the country by area (the largest being Þingeyjarsveit). The biggest town in the municipality is Egilsstaðir, with a population of around 2,500. The second largest town of Seyðisfjörður is one of the oldest towns in Iceland.
The feral reindeer of Iceland are mostly situated in Fljótsdalshérað, where they number between 5,000 and 10,000. The largest forest in Iceland, Hallormsstaðaskógur is also located in Fljótsdalshérað.
The Kárahnjúkar Hydropower Plant is located in the municipality.