Amendeuix-Oneix is located some 50 km east by south-east of Bayonne and 40 km south-west of Orthez in the Mixe country in the former Basque province of Lower Navarre.
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Located in the watershed of the Adour, the northern part of the eastern border of the commune is the Bidouze which flows north to join the Adour west of Peyrehorade.