A Solo 401(k), also called an Individual 401(k) or Self-Employed 401(k), is a retirement savings plan designed for self-employed individuals and small business owners with no full-time employees other than a spouse. It offers the highest contribution limits of any self-employed retirement account: for 2026, you can contribute up to $24,500 as the employee (plus $8,000 catch-up if you're 50+), and up to 25% of net self-employment income as the employer, with a combined maximum of $72,000 ($80,000 age 50+). There is an additional “special” catch up contribution of $3,250 for those 60-63. Contributions can be made pre-tax (traditional) or after-tax (Roth, if the plan allows).