Does Your Solo 401k Need Its Own EIN?

Yes, a Solo 401k needs its own EIN, separate from your Social Security Number and separate from the EIN your business already uses. The plan is legally a trust, and the IRS requires trusts to have their own identifying number for tax purposes.¹ Applying is free, happens on the IRS website, and takes about five minutes.

Why the Plan Trust Needs Its Own Number

A Solo 401k is structured as a qualified trust that holds retirement assets separately from you and from the business that sponsors the plan. Treasury Regulation 301.6109-1 requires any entity other than an individual, trusts included, to use an employer identification number for tax reporting.¹ The trust cannot share your personal SSN or your operating business’s EIN.

The practical side matters just as much. Banks and brokerage firms will not open an account titled in the name of a retirement trust without a dedicated EIN. If contributions and gains are reported under your SSN or business EIN, the IRS has no way to see that the money sits inside a tax-exempt retirement vehicle. That mismatch can produce phantom taxable income and even early withdrawal penalties on money you never touched.

The trust EIN also makes rollovers work. When a former employer’s 401k or a traditional IRA sends funds into your Solo 401k, the receiving custodian reports the transfer under the trust’s EIN. Without one, the transfer can look like a taxable distribution rather than a non-taxable rollover.

What to Have Ready Before You Apply

You apply using IRS Form SS-4.² Finish your plan adoption agreement first, because the EIN application has to match it exactly. The adoption agreement fixes the trust’s legal name, its effective date, and the trustee’s identity.

Have this information on hand when you start:

The trust’s legal name, following the format in the adoption agreement (something like “Smith Consulting 401k Plan Trust”).
Your name and Social Security Number as trustee.
The existing EIN of the business sponsoring the plan.
A mailing address for IRS correspondence.
The plan’s effective date from the adoption agreement.

Two SS-4 Fields That Trip People Up

A retirement plan trust doesn’t fit neatly into the standard entity categories on Form SS-4, and two lines cause most of the confusion. On line 9a (type of entity), check “Other” and write “Created a pension plan.” On line 10 (reason for applying), check “Created a pension plan” and enter the plan type, such as “401k Plan.”³ These entries tell the IRS the EIN belongs to a retirement trust rather than a business, so it doesn’t get saddled with the wrong filing obligations.

Responsible Party

The IRS treats the trustee as the “responsible party” for the trust. That designation carries a follow-up duty: if the responsible party ever changes, you have 60 days to notify the IRS on Form 8822-B.⁴ Common triggers are adding a co-trustee, naming a successor, or restructuring the business in a way that shifts control.

How to Apply

The online IRS EIN application is the fastest route. The system issues the number at the end of the session, so you can open a bank or brokerage account the same day. The tool is available Monday through Friday from 6:00 a.m. to 1:00 a.m. the following morning, Saturday from 6:00 a.m. to 9:00 p.m., and Sunday from 6:00 p.m. to midnight, Eastern Time.⁵ Save or print the confirmation notice before closing the browser. Some custodians ask for a copy of it when opening the account.

If you’d rather use paper, you can fax or mail Form SS-4. Faxing with a return fax number takes about four business days. Mail runs four to five weeks.² Once you have the number, give it to every financial institution holding plan assets so year-end tax documents like 1099-R forms are generated under the trust rather than under you personally.

What the EIN Gets Used For Later

Beyond opening accounts and handling rollovers, the trust’s EIN is what goes on Form 5500-EZ, the annual return for a one-participant plan. Filing is required once the plan’s total assets exceed $250,000 at the end of any plan year. If you maintain more than one Solo 401k under the same business, the IRS combines the balances of all your one-participant plans against that $250,000 threshold, and once it’s crossed, every plan files.⁶

The 5500-EZ is due on the last day of the seventh month after the plan year ends, which is July 31 for a calendar-year plan. A final 5500-EZ is also required for the year you terminate the plan, regardless of the asset balance.⁶ Missing the deadline is costly, so if a filing year approaches, confirm your balance with the custodian before summer.

Updating or Retiring the EIN

Two kinds of changes come up. A change in responsible party or mailing address goes on Form 8822-B within 60 days.⁴ A change to the trust’s legal name is separate: it needs a written request signed by an authorized person and mailed to the IRS office serving your business address, and it cannot be done by phone.⁷ Keep the trust name on IRS records identical to what your custodian has on file.

When you close the plan, the EIN doesn’t disappear. The IRS cannot cancel an EIN but can deactivate it so no further filing obligations attach. Before requesting deactivation, file the final Form 5500-EZ marked as the final return, distribute or roll over all plan assets, and clear any outstanding tax obligations. Then send the IRS a letter with the trust’s EIN, its legal name, the mailing address, a copy of the EIN assignment notice if you have it, and the reason for closing the account, addressed to the IRS in Kansas City, MO 64108 or Ogden, UT 84201.⁸ Once deactivated, the number stays permanently tied to the trust and is never reassigned.

1
GovInfo. Treasury Regulation 301.6109-1 Identifying Numbers
2
Internal Revenue Service. About Form SS-4, Application for Employer Identification Number (EIN)
3
Internal Revenue Service. Instructions for Form SS-4
4
Internal Revenue Service. Form 8822-B, Change of Address or Responsible Party
5
Internal Revenue Service. Get an Employer Identification Number
6
Internal Revenue Service. 2025 Instructions for Form 5500-EZ
7
Internal Revenue Service. Assigning Employer Identification Numbers (EINs)
8
Internal Revenue Service. If You No Longer Need Your EIN