Before you start

Valid passport or national ID — physically required and copied/photographed at point of sale for any SIM (prepaid included), per Greek law.
An unlocked phone or eSIM-capable device (all three carriers offer eSIM).
For postpaid contracts or home internet only: a Greek AFM (tax number), a local address, and usually a Greek/EU bank IBAN for direct debit.

Step-by-step

1
Decide prepaid vs postpaid (the newcomer fork)
If you've just arrived and don't yet have an AFM tax number, go prepaid (καρτοκινητό) — a passport is all you need and you can top up monthly bundles indefinitely. Choose postpaid (συμβόλαιο) only once you have an AFM, a Greek address, and a bank account; it's cheaper per GB and bundles with home internet/TV, but locks you into a 12- or 24-month contract.
OnlineWho: YouSame dayFree (decision)
2
Buy and register a prepaid SIM
Walk into any Cosmote, Vodafone, or Nova store or kiosk in central Athens (Syntagma, Omonia, Athens airport arrivals) with your passport. The agent registers your ID against the SIM on the spot — legally required, takes a few minutes. Starter SIMs cost ~€5; tourist/monthly bundles add data and EU roaming. eSIM is available from all three. Activation is near-instant.
In personWho: You + carrier store/kiosk staff15–30 minutes~€5 SIM; ~€10–20 for a monthly data bundle
3
Top up a monthly bundle (or switch to postpaid)
Recharge prepaid via the carrier app (Cosmote What's Up, My Vodafone, Nova Free2Go), online, ATMs, kiosks, or supermarkets, then activate a bundle: e.g. ~€20 for ~25GB incl. EU roaming + minutes, or smaller ~€13 bundles. To go postpaid later, bring your AFM, address proof and IBAN to a store and sign a plan (~€15–30/mo).
Mobile appWho: YouMinutes (prepaid); same-day in-store (postpaid)~€10–20/mo prepaid; ~€15–30/mo postpaid
4
Order home internet (VDSL or FTTH fibre)
For a flat, order a Double-Play (internet + line) from Cosmote, Vodafone, or Nova online or in-store. You must have an AFM, a Greek address, and sign a contract (usually 12–24 months) with direct debit. Check your address for FTTH fibre (100–1000Mbps) vs older VDSL — fibre rollout in Athens is accelerating but VDSL is still common. A technician visit and line provisioning typically take 1–3 weeks.
OnlineWho: You + carrier + OTE/technician for line install1–3 weeks from order to active line~€25–40/mo; activation/install fee ~€40–55

Documents you’ll need

Passport or national ID (mandatory for all SIMs, copied at sale)
AFM (Greek tax number) — for postpaid plans and home internet
Proof of Greek address, e.g. rental agreement (postpaid / home internet)
Greek or EU bank IBAN for direct debit (postpaid / home internet)

Things most newcomers don’t know

Even a tourist prepaid SIM is registered to your passport — there is no truly anonymous SIM in Greece.

Greek and EU security law requires the seller to record an official ID for every SIM, prepaid καρτοκινητό included; an unregistered SIM gets suspended. Bring your passport, not just cash.

Source: Greek SIM registration law / EETT framework

A Greek prepaid SIM works across the entire EU at no surcharge — buy local, roam continental.

Under EU 'Roam Like at Home' rules, your Greek bundle's data, calls and texts work in every EU/EEA country at domestic rates. For an Athens-based expat who travels in Europe, a cheap Greek prepaid bundle doubles as a continental SIM (subject to a roaming fair-use data cap).

Source: EU Roaming Regulation / EETT

The real blocker for cheaper postpaid plans and home fibre is the AFM, not the carrier.

Postpaid (συμβόλαιο) and home internet require an AFM, a Greek address and usually a Greek IBAN for direct debit. Run a prepaid bundle for your first weeks and only switch once you've obtained an AFM from the DOY/AADE and opened a bank account.

Source: Carrier postpaid signup requirements (Cosmote/Vodafone/Nova)

Common mistakes to avoid

Buying a SIM without your passport — you can't; ID registration is mandatory and done at the counter for prepaid too.
Assuming you can sign a monthly contract on arrival — postpaid and home internet need an AFM, address proof and usually a Greek IBAN you won't have yet.
Letting a prepaid SIM go unused for too long — numbers can be deactivated after a long period with no top-up or usage.
Expecting fibre everywhere — many Athens addresses are still VDSL, not FTTH; check the exact street/building before ordering a high-speed plan.
Underestimating home-internet install time — line provisioning and a technician slot can take 1–3 weeks, so don't rely on it for day-one connectivity.