Otoyol 21 (English: Motorway 21), abbreviated as O-21, a.k.a.

Ankara-Tarsus Otoyolu (English: Ankara-Tarsus Motorway), is a completed toll motorway in the Central Anatolia and Mediterranean regions in Turkey.

Currently, it connects Otoyol 20 from Gölbaşı, Ankara with the Adana-Mersin Motorway O-51 at Tarsus in Mersin Province.

On its full length, it is part of the European E90.

[1] With a length of 432 kilometres, the O-21 is the longest motorway in Turkey.