An FEIN, or Federal Employer Identification Number, is a unique nine-digit number assigned by the Internal Revenue Service (IRS) to business entities operating in the United States for the purposes of identification. It is also known as a Federal Tax Identification Number or a Taxpayer Identification Number (TIN) . An FEIN is used for tax administration and must not be used for any other purpose. It is usually written in the form 00-0000000 to differentiate it from a Social Security Number (SSN), which is usually written in the form 000-00-0000.

An FEIN is required for businesses that have employees and/or are incorporated. Sole proprietorships that do not have employees can use the business owners SSN as a tax ID. Businesses that typically do not need an FEIN include those that do not have employees, do not file tax returns related to employment, excise (sin tax), alcohol, tobacco or firearms, do not withhold taxes on income paid to a non-resident alien, and do not have a Keogh plan or tax-deferred pension plan.

To obtain an FEIN, businesses can apply directly through the IRS, which usually issues them immediately. Applicants must fill out Form SS-4: Application for Employer Identification Number, which is available on the IRS website. The form requires information such as the name of the companys principal officer, partner, trustor, owner, or any other title, along with their personal taxpayer identification number.