Usually yes — but it’s the same EIN as your business’s, not a separate “plan EIN.” The IRS requires the employer’s EIN on your Solo 401(k)’s required paperwork and won’t accept a Social Security number there. For the typical one-owner plan, that’s the only EIN involved — there isn’t a second, plan-specific one to track down.
Why you need one at all: the plan, not necessarily the business
If you’re a sole proprietor with no employees, you may not need an EIN for the business by itself. The Schedule C instructions spell out when you do: “You need an EIN only if you have a qualified retirement plan or are required to file employment, excise, alcohol, tobacco, or firearms returns or are a payer of gambling winnings,” per the IRS Instructions for Schedule C, Line D. A Solo 401(k) is exactly that trigger — having one is enough to need an EIN, even if nothing else about a one-person business would require it.
It’s your business’s EIN — not a separate one for the plan
This is the part that gets overstated: a Solo 401(k) doesn’t need its own, distinct “plan EIN” separate from your business. The Instructions for Form 5500-EZ — the return one-participant plans generally file once plan assets reach $250,000 — say to “enter the employer’s nine-digit employer identification number (EIN)… Do not enter a social security number (SSN)” on the line for the plan sponsor. There’s also a line for the plan administrator’s EIN, but the instructions are explicit: “unless the administrator is the employer identified [on the employer line]… enter the word ‘Same’… and leave the remainder… blank.” For a typical Solo 401(k), you’re both the employer and the administrator, so you write “Same” and that’s it — one EIN, used for both roles.
Where this EIN actually gets used
- Schedule C, Line D. Once you have the plan, this is the EIN you enter on your own tax return.
- Form 5500-EZ, if your plan’s assets ever reach $250,000 — the annual filing threshold for one-participant plans, per the IRS page on Form 5500-EZ.
- Opening the account with a provider. This part varies. Some Solo 401(k) providers ask for your EIN upfront; others will let a sole proprietor with no employees start the process without one and only need it later. Either way, it’s the same business EIN you’d use everywhere else — not something separate to apply for on the plan’s behalf.
How to get one
Applying for an EIN is free and direct through the IRS — you can apply online and typically receive the number immediately, per the IRS page on employer identification numbers. You don’t need to pay a third party to do this for you.
What if I already have a business EIN?
Then you’re already set for the plan, too. Since the Form 5500-EZ instructions treat the plan administrator’s EIN as the same as the employer’s whenever one person fills both roles, there’s no separate application to make. If a provider specifically asks you for “the plan’s EIN,” they generally mean your business’s EIN, not a second number.
Bottom line
Don’t assume “I’m self-employed with no employees, so I don’t need an EIN” settles the question — having a Solo 401(k) is its own trigger for needing one. But once you have that EIN, it does double duty: your business and your retirement plan share it, rather than the plan needing a second one of its own.