Reitnau is first mentioned in 1045 as "Reitinouwa". The village was originally owned by the Count of Lenzburg, though in 1045 it and the village church were granted to the convent in Schänis. The property was administered by a representative of the convent. Following the conquest of the Aargau in 1415, the village belonged to Bern and was part of the bailiwick of Lenzburg. However, the low court for Reitnau continued to be held in Schänis. It remained part of Bern until 1798 and in 1803 became a municipality in the new canton of Aargau. In 1873 the municipality opened its first schoolhouse, and in 1905, it received a district school.