Transport for Athens (Greek: Συγκοινωνίες Αθηνών, romanized: Sygkoinonies Athinon), officially the Athens Urban Transport Organisation (Greek: Οργανισμός Αστικών Συγκοινωνιών Αθηνών, Organismos Astikon Sygkoinonion Athinon, ΟΑΣΑ), is the operator of public transport in Athens, Greece. Transport for Athens, through its subsidiary companies, operates metro, tram, trolleybus and bus services in the Athens metropolitan area.

As of July 2011, the Athens Mass Transit System consists of:

Transport for Athens also coordinates the Athens Suburban Railway, using Hellenic Railways Organisation (OSE) lines, operated by Hellenic Train S.A. under the Proastiakos brand. The section between Piraeus, Magoula and Koropi is regarded as the urban part.

In March 2011, with the Greek Government Law 3920 Attiko Metro Operational Company (AMEL S.A.) absorbed Athens-Piraeus Electric Railways S.A. and TRAM S.A. and was renamed "STASY S.A." (Greek: ΣΤΑΣΥ Α.Ε.). Also ETHEL S.A. absorbed ILPAP S.A. and was renamed as "OSY S.A." (Greek: ΟΣΥ Α.Ε.). The mergers were officially announced on June 10, 2011. While mergers at the top management level took place quickly, integration of the former companies at operations and support level proceeds slowly.

ISAP, ETHEL and ILPAP were wholly owned by OASA. AMEL and Tram S.A. until June 2011 were subsidiaries of Attiko Metro S.A. (Greek: Αττικό Μετρό Α.Ε.), a company that also wholly owned by the Greek government.

Hellenic Train (formerly Trainose until May 2022) S.A., which absorbed the former Proastiakos S.A. in 2007, became independent of the OSE group in 2008 and until 2017 it is a separate private company.

OASA regularly conducts street photography and videography competitions both in black-and-white and in colour for photos taken while inside its means of transport.

Up until the 80s, fare collectors were commonplace in all means of mass transport in Athens. Their replacement was a money box placed at the front end of the vehicles, next to the driver, where passengers would deposit their fare in cash, usually coinage. A decade later, they were replaced with validation machines and paper tickets. The end of the single-use paper ticket came in 2017, when it was succeeded by the ATH.ENA electronic stored-value cards and tickets (stylized as ATH.ENA CARD and ATH.ENA TICKET respectively). Tickets are now rechargeable and can be loaded with up to 5-day unlimited ticket, but not with long term fare products, discount tickets (support discontinued in January 2019) or monetary value. Cards (personalised and anonymised) can hold all fare types and a maximum of €50, with the exception of discount tickets which are tied to personalised cards.