Otoyol 21, abbreviated as O-21, a.k.a. Ankara-Tarsus Otoyolu, is a completed toll motorway in the Central Anatolia and Mediterranean regions in Turkey. Currently, it is connecting 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. The last part of the highway opened on 16 December 2020.

European route E 90 is an A-Class West–East European route, extending from Lisbon in Portugal in the west to the Turkish–Iraqi border in the east. It is connected to the M5 of the Arab Mashreq International Road Network.