Do I Need an EIN for a Solo 401(k)? Rules and How to Apply

Yes, you need an EIN for a Solo 401(k). The plan’s trust is a separate legal entity from you and from your business, so the IRS requires it to have its own nine-digit Employer Identification Number. The EIN is free, and the fastest way to get one is through the IRS online EIN assistant, which issues the number at the end of a single session.1Internal Revenue Service. Get an Employer Identification Number

Why the Plan Needs Its Own EIN

A Solo 401(k), sometimes called a one-participant 401(k), is a traditional 401(k) for a business owner with no employees other than a spouse.2Internal Revenue Service. One-Participant 401k Plans Federal tax law requires every qualified retirement plan to hold its assets in a trust created for the exclusive benefit of the participants.3Office of the Law Revision Counsel. 26 USC 401 – Qualified Pension, Profit-Sharing, and Stock Bonus Plans That trust is its own entity, and the IRS needs a way to identify it apart from you and your business.

The plan EIN is that identifier. The IRS uses it to verify that contributions going into the trust are tax-deferred or Roth, that investment earnings inside the trust are not currently taxed, and that distributions are reported correctly. Banks and brokerages use the same number to set the account up as a tax-exempt retirement trust rather than a personal or business account.1Internal Revenue Service. Get an Employer Identification Number

Can You Use Your Business EIN or Social Security Number Instead?

No. If your business already has an EIN, that number identifies your business for payroll taxes, income tax filings, and vendor payments. The Solo 401(k) trust needs a different EIN. Using the business EIN on plan paperwork can trigger reporting errors, delays in opening accounts, and complications during IRS audits.

Some brokerage firms will let a sole proprietor open a Solo 401(k) account using just a Social Security Number. Even in that case, the plan still needs its own EIN to file Form 5500-EZ when required and to keep the trust’s finances cleanly separated from your personal and business records.4Internal Revenue Service. Financial Advisors – Are Assets in Your Clients One Participant Plans More Than 250000 Getting the number up front is easier than untangling accounts later.

What to Have Ready Before You Apply

Finalize your plan adoption agreement first. The adoption agreement is the legal document that creates your Solo 401(k), and it defines the plan’s name, the trustee, contribution types, and other terms. Several fields on the EIN application pull directly from it, so completing it first prevents mismatched information later.

The EIN application uses IRS Form SS-4. Have the following ready:5Internal Revenue Service. Instructions for Form SS-4 (Rev. December 2025) – Application for Employer Identification Number

  • The legal name of the trust, typically formatted like “Jane Smith Solo 401k Trust.” It should match the name in your adoption agreement.
  • The trustee’s name for Line 3. As a Solo 401(k) owner, you are normally the plan trustee.
  • The responsible party’s name and Social Security Number for Lines 7a and 7b. This is the trust’s grantor, owner, or trustor, which is you.
  • On Line 9a, check “Other” and write “Created a pension plan.”
  • On Line 10, check “Created a pension plan” and enter the plan type (for example, “401k”).
  • A mailing address where the IRS should send correspondence about the plan.

How to Apply for the EIN

The IRS offers three ways to apply, each with a different turnaround.6Internal Revenue Service. Employer Identification Number

Online

The fastest option is the IRS online EIN assistant at irs.gov. You answer a series of questions, and the system issues your EIN at the end of the session. The tool is available Monday through Friday from 6:00 a.m. to 1:00 a.m. Eastern, Saturday from 6:00 a.m. to 9:00 p.m. Eastern, and Sunday from 6:00 p.m. to midnight Eastern. There is no fee.1Internal Revenue Service. Get an Employer Identification Number

Fax

Fax a completed Form SS-4 to 855-641-6935. If you include a return fax number, the IRS will fax back a cover sheet with your EIN in about four business days.6Internal Revenue Service. Employer Identification Number

Mail

Mail the completed Form SS-4 to Internal Revenue Service, Attn: EIN Operation, Cincinnati, OH 45999. Expect the EIN in about four weeks.6Internal Revenue Service. Employer Identification Number

After the IRS Assigns Your EIN

Once your EIN is issued, the IRS mails a CP 575 confirmation notice. The notice lists the new EIN, identifies any returns the plan is required to file, and shows the first due date for each return.7Internal Revenue Service. Assigning Employer Identification Numbers Keep the CP 575 in your permanent records. The IRS cannot reissue the original; if you lose it and need verification later, you will receive a different document (Letter 147C) instead.

With the EIN in hand, open a dedicated bank or brokerage account in the name of the trust. Financial institutions use the EIN to set the account up as a tax-exempt retirement trust, which determines how contributions, earnings, and distributions are reported. Once the account is open, you can start depositing contributions and investing plan assets.