Otoyol 31 (English: Motorway 31), abbreviated as , aka İzmir-Aydın Otoyolu (English: Izmir-Aydın Motorway), is a tollmotorway in the Aegean Region of Turkey connecting the cities of İzmir and Aydın.
The motorway links to the beltway of Izmir on its northern end and connects to the state road bypassing Aydın in the south. On its total length of 108km (67mi), the O-31 connects to the European route .
Located at the province border of Izmir and Aydın between Belevi and Germencik junctions and went in service on April 20, 2000, the 75th Anniversary Selatin Tunnel (Turkish:75. Yıl Selatin Tüneli), is with its length of 3,043m (9,984ft) Turkey's second longest road tunnel.