What is TIN (Taxpayer Identification Number)?

How it works

"TIN" is the IRS's generic word for "any tax identification number". On forms (W-9, W-8, 1099, 1042-S, 1040, 1120) the field labelled "TIN" accepts whichever sub-type applies to you:

TIN type	Who	Issued by	Application
SSN	US-work-authorized individuals	Social Security Administration	Form SS-5
ITIN	Individuals not eligible for SSN	IRS	Form W-7
EIN	Entities (LLC, corp, partnership, trust)	IRS	Form SS-4
ATIN	Adopted children pending SSN	IRS	Form W-7A
PTIN	Paid tax preparers	IRS	online registration

ATIN and PTIN are niche. The three that matter for cross-border founders are SSN, ITIN, and EIN.

Picking the right TIN

The decision tree is simple:

Are you applying as a person, or as a business? Person → SSN or ITIN. Business → EIN.
As a person: are you eligible for an SSN? Yes (citizen, green card, work-authorized) → SSN. No → ITIN.

Practical implications for non-residents:

A foreign-owned single-member LLC has an EIN (the entity) but the owner may also need an ITIN to file Form 1040-NR or claim treaty rates on W-8BEN. The two TINs are separate and both may be required.
A foreign founder with a US C-corp typically only deals with the corporation's EIN. The shareholder doesn't need a personal TIN unless they receive dividends and want treaty rates on a W-8BEN.

What "TIN" never means

Not a foreign tax ID. The IRS field labelled "TIN" specifically means a US-issued one.
Not a SIRET / SIREN, NIF, NIE, RFC, CPF, NHS number, or any other national ID.
Not the GIIN — that's a FATCA-specific identifier for financial institutions.

Examples

French founder + Wyoming SMLLC. Two TINs in play: EIN for the LLC (issued via SS-4 fax), and ITIN for the founder (issued via W-7) if he claims treaty rates on a W-8BEN. He doesn't need an SSN.
US citizen running a Schedule C business. One TIN: his SSN. The IRS treats it as the universal identifier. He could also obtain an EIN to use on 1099s issued to vendors instead of his SSN, for privacy reasons — but the SSN remains his personal TIN.

Common mistakes

Mixing personal and entity TINs. A single-member LLC owned by a US person files W-9 with the owner's SSN/EIN; the LLC's own EIN is for banking and 1099s but not the W-9 to a US client. Foreign-owned SMLLCs file W-8BEN with the owner's foreign info — the LLC's EIN doesn't appear on a W-8.
Trying to use a foreign tax ID on US forms. US payers reject any non-US tax ID in the TIN field. The form is voided and 30% withholding applies.
Letting an ITIN expire and assuming the SSN/EIN still work. They do — but a stale ITIN blocks personal filings and treaty claims even if the entity-side EIN is fine.
Confusing TIN with the GIIN (FATCA). GIIN is for foreign financial institutions reporting under FATCA; it's never a substitute for SSN/EIN/ITIN in normal payee onboarding.

Frequently asked questions

Which TIN do I need?

Individual + US-work-authorized → SSN. Individual + non-US (or US person without SSN) → ITIN. Entity (LLC, corp, partnership) → EIN.

Can I use my home country's tax ID instead?

No — US payers and the IRS only accept US-issued TINs. Treaty claims on W-8BEN require a US TIN to apply the reduced rate.

Do I need a TIN to open a US bank account?

Yes. Mercury, Brex, Wise and others require an EIN for the LLC and either an SSN, ITIN, or passport for the individual signer.

Are foreign tax IDs ever recognized?

Indirectly. A foreign tax-residency certificate (issued under your home country's TIN system) can support treaty claims, but the W-8BEN itself still asks for a US TIN if a treaty rate is being invoked.