| Montagnieu | |
| Commune Status: | Commune |
| Arrondissement: | Belley |
| Canton: | Lagnieu |
| Insee: | 01255 |
| Postal Code: | 01470 |
| Mayor: | Jean Roset[1] |
| Term: | 2020 - 2026 |
| Coordinates: | 45.7947°N 5.4681°W |
| Elevation M: | 225 |
| Elevation Min M: | 200 |
| Elevation Max M: | 714 |
| Area Km2: | 6.22 |
Montagnieu (in French pronounced as /mɔ̃taɲø/) is a commune in the Ain department in eastern France. It is sited on the eastern bank of the river Rhone, which forms the border with the Isere department to the west.
The 18th century parish church is dedicated to Saint Didier.
Montagnieu has an oceanic climate (Köppen climate classification Cfb). The average annual temperature in Montagnieu is . The average annual rainfall is with November as the wettest month. The temperatures are highest on average in August, at around, and lowest in January, at around . The highest temperature ever recorded in Montagnieu was on 13 August 2003; the coldest temperature ever recorded was on 7 February 2012.