Andrein is a French commune in the Pyrénées-Atlantiques department in the Aquitaine region of south-western France.
The inhabitants of the commune are known as Andreinais or Andreinaises
Andrein is a béarnaise commune located on the left bank of the Gave d'Oloron 5 kilometres east of Sauveterre-de-Béarn and some 16 km south-west of Orthez. Access to the commune is by road D27 from Sauveterre-de-Bearn passing through the commune and the village and continuing east to Laàs. The D23 road from Burgaronne to L'Hôpital-d'Orion also passes through the north of the commune. The commune is mixed forest and farmland.
Located in the Drainage basin of the Adour, the southern border of the commune is formed by the Gave d'Oloron. Numerous streams flow south through the commune to the Gave d'Oloron including the Malourau and the Lourou which forms the eastern border. The northern border is formed by the Arrec Heurre which flows west to eventually join the Gave d'Oleron east of Abitain.