Before you start

Passport — required for every SIM purchase, tourist or resident
A second ID — for residents: your ARC; for tourists: entry permit/landing slip, NHI card, driving licence, or a second photo ID
Age 20+ to buy a SIM in your own name (under-20s need a guardian)
For postpaid contracts: an ARC, and sometimes proof of local address and/or a Taiwan bank account or credit card for the monthly autopay

Step-by-step

1
On arrival: grab a tourist prepaid SIM or eSIM at Taoyuan
Chunghwa, Taiwan Mobile and FarEasTone all run staffed counters in the Taoyuan (TPE) arrival halls. Show passport + one other ID and you walk out connected in ~10 minutes. Day-pass bundles run roughly NT$300 (~US$9, ~5 days) to NT$500-600 (~US$15-19, 7-10 days unlimited), up to ~NT$1,000 (~US$31) for 30-day unlimited. Chunghwa also sells a tourist eSIM at the counter.
In personWho: You~10 minutes on arrivalNT$300-1,000 (~US$9-31) depending on days/data
2
Once you have your ARC: switch to a resident prepaid or postpaid plan
With an ARC you can buy a normal prepaid SIM at any carrier store (passport + ARC = your two IDs) or sign a postpaid contract. Prepaid monthly data packs are cheap; postpaid 5G plans run ~NT$499-999/mo for generous data and ~NT$1,399+/mo for genuinely unlimited 5G. Postpaid usually needs the ARC plus, at some carriers, proof of address or a local bank card for autopay.
In personWho: YouSame day at a carrier storePrepaid packs from ~NT$300/mo; postpaid NT$499-1,399+/mo
3
Pick a carrier for coverage vs price
Chunghwa Telecom (中華電信) has the widest, most reliable coverage including rural/island areas — the default if you travel around Taiwan. Taiwan Mobile (台灣大哥大) and FarEasTone (遠傳, now merged with Asia Pacific/T-Star) are strong in metro Taipei and often a touch cheaper. 5G is near-universal in the city; for Taipei-only use any of the three is fine.
In personWho: YouSame dayIncluded in plan choice
4
Set up home broadband (HiNet fibre) once you have a lease
Chunghwa's HiNet fibre is the dominant home internet — order online or in-store; installation needs your ARC and lease/address. Plans run ~NT$499-599/mo (100M) up to NT$999-1,399/mo (gigabit); a one-off install/line fee (~NT$1,500) is often waived or discounted on a 2-year contract. Many Taipei rentals already include shared fibre — check before ordering your own line.
OnlineWho: You (with landlord/lease)A few days to schedule an engineerNT$600-1,200/mo typical; install fee often waived on contract

Documents you’ll need

Passport (every purchase)
ARC (residents — this is your second ID and unlocks postpaid + fibre)
For tourists: entry permit/landing slip, NHI card, driving licence, or other photo ID as the second ID
Proof of local address / lease (postpaid contracts and home broadband)
Taiwan bank card or credit card (some postpaid autopay setups)

Things most newcomers don’t know

The 'two-ID rule' is the thing that trips newcomers — and your ARC is the magic second ID.

Taiwan law requires two forms of ID to register any SIM. Before your ARC arrives, your passport + entry permit/NHI card/second photo ID work for a tourist prepaid SIM; but you can't sign a postpaid contract until the ARC exists. Time your contract for after the ARC, not before.

Source: NCC / carrier consensus

Unlimited 5G is real but pricey — most residents are happier on a mid-tier capped plan.

Truly unlimited 5G postpaid starts around NT$1,399/mo, roughly double a generous capped plan (~NT$499-999). Taiwan's data is cheap and fast enough that a capped plan rarely bites; only heavy tetherers/streamers need the unlimited tier.

Source: Taiwan Mobile rate plans

Your rental may already include HiNet fibre — don't pay twice.

Many Taipei apartments come with shared or landlord-provided fibre bundled into rent. Ordering your own HiNet line means an install fee and a 2-year contract you may not need. Ask the landlord what's already wired before signing up.

Source: provider / expat consensus

Common mistakes to avoid

Assuming one ID is enough — every SIM needs two; without your ARC, bring a real second photo ID/entry permit
Trying to sign a postpaid contract before your ARC is issued — you'll be limited to prepaid
Buying an unlimited NT$1,399+ plan you don't need when a NT$499-999 capped plan is plenty
Ordering your own HiNet line when the apartment already includes fibre
Forgetting postpaid and fibre are 24-month contracts with early-termination fees