Before you start

A valid passport or national ID (ID may be requested to register the line)
A French RIB / IBAN for the monthly direct debit on most postpaid plans — or use a prepaid SIM if you don't have one yet
A French address for home internet (the 'box') and an address eligibility check before you can order
An unlocked phone (eSIM is widely supported by all four networks)

Step-by-step

1
Day one: prepaid SIM, or a SIM-only plan if you have a card it accepts
If you have no French bank account yet, buy a prepaid SIM (carte prépayée) from Orange, SFR, Lebara or Lycamobile at an airport kiosk, carrier shop, tabac or supermarket — no RIB, no commitment. If you can pay with a foreign/EU card, several no-commitment SIM-only plans (Free, Bouygues B&You, Red by SFR) will take it. eSIM is supported, so you can often activate before landing.
In personWho: YouSame dayPrepaid starter ~€10-20 incl. credit; SIM-only plans ~€10-20/mo
2
Pick a SIM-only no-commitment plan once your bank account is open
With a French RIB you unlock the cheap postpaid market. Sosh (Orange), Red by SFR and B&You (Bouygues) sell ~100-200GB 5G for ~€10-20/month, all sans engagement. Free Mobile's headline plan is unlimited-ish (300GB+) 5G at €19.99/month — and just €2/month if you also have a Freebox at home. MVNOs like Prixtel and Lebara compete on price. Billing is by monthly prélèvement from your RIB.
OnlineWho: YouActivates within minutes to ~24h~€10-20/mo; Free €19.99/mo unlimited or €2/mo for Freebox subscribers
3
Keep your number: port it with a RIO code
To move carriers and keep your number, call 3179 (free) from the line you want to keep to get your RIO code, then give that code to the new operator at sign-up — they handle the port and cancel the old line for you. Porting is free and your number transfers in roughly 1-3 working days with minimal downtime. This works from prepaid to postpaid too.
Mobile appWho: You and the new operator~1-3 working daysFree
4
Set up home internet — order a fibre 'box'
Run an address eligibility check (test d'éligibilité) on Orange (Livebox), Free (Freebox), SFR or Bouygues (Bbox) — fibre (FTTH) is widespread across Paris. Order online with a French address and usually a RIB for the direct debit, then book an installation appointment for the technician. Expect ~€30-45/month after intro pricing.
OnlineWho: YouInstall appointment ~1-2 weeks~€30-45/mo (intro promos often cheaper for 12 months)

Documents you’ll need

Valid passport or national ID (may be requested to register the line)
A French RIB / IBAN for the direct debit (most postpaid plans and home internet)
A French address (required for home internet; used in the eligibility check)
A foreign/EU bank card (accepted by some no-commitment plans if you have no RIB yet)

Things most newcomers don’t know

Most postpaid plans bill by direct debit from a French RIB — which is why new arrivals start prepaid.

French SIM-only plans collect the monthly fee by prélèvement (SEPA direct debit) from a French bank account you won't have on day one. A prepaid SIM or a plan that accepts a foreign EU card bridges the gap until your account is open, then you port the number over.

Source: carrier sign-up terms (Sosh, Red by SFR, B&You, Free)

Free Mobile's plan drops to €2/month if you also have a Freebox at home.

Free bundles its products: the headline €19.99/month unlimited mobile plan costs only €2/month for Freebox home-internet subscribers. If you're setting up both anyway, taking the Freebox first makes the mobile line almost free — a deliberate lock-in worth doing the maths on.

Source: Free Mobile (mobile + Freebox offers)

A French SIM roams across the EU at no extra cost.

Under EU 'roam like at home' rules, your French plan's calls, texts and data work in every EU/EEA country with no surcharge, using your normal allowance — a cheap French SIM doubles as a pan-European line (fair-use data caps apply abroad).

Source: ARCEP / EU roaming regulation

Common mistakes to avoid

Trying to sign up for a postpaid plan with no French RIB — most need one for the direct debit; start prepaid instead
Ordering home internet before running the address eligibility check (test d'éligibilité)
Switching carriers without requesting your RIO code first (call 3179) and losing your number
Overlooking that home-internet 'box' contracts are typically 12-month commitments, unlike no-commitment mobile plans

Some of this may be out of date. Spotted something inaccurate? Help us keep it right for the next newcomer.