Montagnieu (French pronunciation: [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 12.4 °C (54.3 °F). The average annual rainfall is 1,099.9 mm (43.30 in) with November as the wettest month. The temperatures are highest on average in August, at around 21.5 °C (70.7 °F), and lowest in January, at around 3.2 °C (37.8 °F). The highest temperature ever recorded in Montagnieu was 40.0 °C (104.0 °F) on 13 August 2003; the coldest temperature ever recorded was −14.7 °C (5.5 °F) on 7 February 2012.