Yes, Illinois has a state income tax that applies to most residents and workers

Illinois taxes income earned within the state at a flat rate of 4.95 percent. This rate applies to wages, salaries, interest, dividends, and other forms of income. The tax is withheld from paychecks by employers, similar to federal income tax withholding.

The state also taxes retirement income, including distributions from 401(k) plans, IRAs, and pensions, though some retirement income receives partial or full exemption depending on your age and the source of the funds. Military pensions and certain public employee pensions have different rules than private retirement accounts.

Unlike the federal income tax system, Illinois does not use tax brackets. Everyone pays the same 4.95 percent rate regardless of how much they earn. This means a person making $30,000 per year pays the same percentage as someone making $300,000 per year.

Key Takeaways

  • Illinois charges a flat 4.95 percent state income tax on wages, salaries, and most other income sources.
  • Your employer withholds state income tax from your paycheck automatically, just as they do for federal tax.
  • Retirement income is taxed, but military pensions and some public employee pensions are exempt from state tax.
  • You file a state income tax return with the Illinois Department of Revenue if you earned income in Illinois during the tax year, even if no tax is owed.

Who must file a state income tax return in Illinois

You must file an Illinois state income tax return if you lived in Illinois for any part of the tax year and earned income there. This includes W-2 wages, self-employment income, rental income, and investment income. The requirement to file exists even if your total income falls below the threshold where you would owe tax.

Nonresidents who earned income in Illinois during the year must also file a state return, even if they live in another state. This applies to people who worked in Illinois temporarily, had rental property in the state, or received other Illinois-source income.

If you are claimed as a dependent on someone else's return, you may still need to file your own Illinois return if your income exceeds certain thresholds. The threshold depends on the type of income you received.

How Illinois state income tax withholding works

Your employer deducts Illinois state income tax from each paycheck based on information you provide on Form IL-W-4, the Illinois withholding certificate. This form asks for your filing status, number of dependents, and other personal information that determines how much tax is withheld. You complete this form when you start a job, and you can update it if your situation changes.

The amount withheld is sent to the Illinois Department of Revenue by your employer, usually along with federal withholding and Social Security taxes. At the end of the year, your employer reports the total amount withheld on your W-2 form.

If too much tax was withheld during the year, you receive a refund when you file your return. If too little was withheld, you owe the difference. You can adjust your withholding at any time by submitting a new Form IL-W-4 to your employer's payroll department.

Retirement income and pension tax treatment in Illinois

Illinois exempts military pensions entirely from state income tax. This means if you receive a pension from the U.S. military, that income is not subject to the 4.95 percent state tax.

Public employee pensions — those paid to retired teachers, police officers, firefighters, and other government workers — receive different treatment depending on when the pension payments began. Pensions that started before January 1, 2011 are exempt from state tax. Pensions that started on or after January 1, 2011 are taxed at the 4.95 percent rate.

Private retirement accounts, including traditional IRAs, 401(k) plans, and Roth IRAs, are subject to state income tax when you withdraw money. The same applies to distributions from SEP-IRAs and SIMPLE IRAs. Social Security benefits are not taxed by Illinois.

Self-employment income and the state tax

If you are self-employed or own a business in Illinois, you must pay state income tax on your net business income. This is the income remaining after you subtract business expenses from gross revenue. You report this income on your Illinois state return.

Self-employed people do not have an employer withholding tax, so you may need to make estimated tax payments to the Illinois Department of Revenue four times per year. These payments are due in April, June, September, and January. If you do not make estimated payments and owe a large amount at tax time, you may face penalties and interest.

You can avoid estimated payments if your expected tax liability is less than $150 for the year. Many self-employed people work with a tax professional or use tax software to calculate whether estimated payments are required.

Filing your Illinois state income tax return

You file your Illinois return with the Illinois Department of Revenue, separate from your federal return. The state return uses information from your federal return, including your total income and filing status. Most people file both returns at the same time using tax software or a tax professional.

The deadline to file is typically April 15, the same date as the federal deadline. If you file for a federal extension, your Illinois return is also extended. You can file electronically or by mail; electronic filing is faster and reduces errors.

If you owe state tax, you can pay online through the Department of Revenue website, by mail, or through your tax software. If you are due a refund, the state processes it within four to six weeks for electronic returns and longer for paper returns.

Nonresident income and part-year residents

If you moved to or from Illinois during the tax year, you file as a part-year resident. You report only the income you earned while living in Illinois on your state return. Income earned before you moved to Illinois or after you moved away is not reported to Illinois, though it may be reported to the state where you lived at that time.

Nonresidents who earned income in Illinois — such as someone who worked a temporary job in the state or owned rental property there — must file an Illinois return reporting only that Illinois-source income. They do not report income earned in their home state on the Illinois return.

The Illinois Department of Revenue provides a worksheet to help part-year residents and nonresidents calculate which income to report. Your tax software typically handles this calculation automatically if you enter your residency status correctly.

Frequently Asked Questions

Do I have to pay Illinois state income tax if I work in Illinois but live in another state?

Yes. Illinois taxes income earned within the state, regardless of where you live. You file an Illinois return reporting your Illinois wages and also file a return in your home state. Some states offer a credit for taxes paid to other states to prevent double taxation, so check your home state's rules.

Is Social Security taxed by Illinois?

No. Illinois does not tax Social Security benefits. If Social Security is your only income, you do not need to file an Illinois return. If you have other income sources, you report those but not the Social Security amount.

What happens if I don't file an Illinois state return?

The Illinois Department of Revenue can assess penalties and interest on any tax owed. If you are due a refund, you forfeit it if you do not file within a certain time period. Filing even when you owe no tax keeps your record current with the state.

Can I claim dependents on my Illinois return to reduce my tax?

No. Illinois does not allow personal exemptions or dependent deductions on the state return. The flat 4.95 percent rate applies to your income regardless of dependents or filing status. Your filing status affects withholding but not the tax rate itself.

What if my employer did not withhold Illinois state tax from my paycheck?

Contact your employer's payroll department immediately. They may have made an error on your Form IL-W-4 or failed to process it. If the error is not corrected, you will owe the unpaid tax when you file your return, plus potential penalties. Documenting the error in writing protects you if the Department of Revenue questions the missing withholding.