What is IRS Form 6166?

What is Form 6166?

Form 6166 (officially "Certification of US Tax Residency") is the US tax residency certificate that proves you qualify for reduced tax rates under international tax treaties.

When you need it:

If you're a US resident earning income abroad, foreign tax authorities may require Form 6166 to apply treaty benefits—like reduced withholding rates—instead of the higher standard rates for non-residents.

Example: A US engineer works temporarily in Norway for a Norwegian company. Without Form 6166, Norway would withhold taxes at their standard non-resident rate. With the certificate, the engineer can claim the lower US-Norway treaty rate.

Note: doola does not currently assist with Form 6166 applications.