Greek National Road 91 (), abbreviated as the EO91, is a national road in Attica, Greece. The EO91 runs along the western face of the Attica peninsula, from Athens to Sounion.
The EO91 is officially defined as a north-south coastal road in Attica. From Athens in the north, the road travels southwest along Andrea Syngrou Avenue towards Palaio Faliro: from there, the EO91 follows the coast of the Saronic Gulf to Sounion in the south, passing through Vouliagmeni. The Athens Tram runs parallel to the EO91, from Palaio Faliro to Glyfada.
The EO91 connects with the EO1, EO54 and Vouliagmenis Avenue in Athens, and the EO89 at Sounion.
Ministerial Decision G25871 of 9 July 1963 created the EO91 from the old EO3, which existed by royal decree from 1955 until 1963, and followed the same route as the current EO91.