Yes — open enrollment is required by law, but the requirement attaches to specific coverage systems rather than to you as an individual. The Affordable Care Act directly requires health insurance marketplaces to hold an annual open enrollment period. Federal tax and benefits rules effectively force employers who offer health coverage to run one as well. Medicare operates its own statutory enrollment calendar with permanent penalties for missing it. Medicaid and CHIP, by contrast, have no enrollment window at all.
The ACA Marketplace Requirement
The Affordable Care Act directed the federal government to create health insurance marketplaces and required those marketplaces to establish annual open enrollment periods. The specific dates are set each year by the Secretary of Health and Human Services.1Office of the Law Revision Counsel. 42 USC 18031 – Affordable Choices of Health Benefit Plans For the 2026 plan year, the window runs November 1 through January 15.2HealthCare.gov. Get Health Insurance Answers
When you enroll inside that window matters. Sign up by December 15 and your coverage starts January 1. Sign up between December 16 and January 15 and coverage begins February 1.
Outside the window, you generally cannot buy a marketplace plan unless a qualifying life event opens a Special Enrollment Period for you. The deadline exists partly to keep people from waiting until they’re sick to buy in, which would push premiums up for everyone else.
What the Law Requires of Employers
Federal law does not require every employer to offer health insurance. What it does is put heavy financial pressure on larger employers to offer it, and then regulate how any employer that does offer coverage must run the plan.
Under Internal Revenue Code Section 4980H, an applicable large employer — one that averaged at least 50 full-time employees during the prior calendar year — owes a penalty if it fails to offer affordable coverage that provides minimum value to substantially all of its full-time workers.3Office of the Law Revision Counsel. 26 USC 4980H – Shared Responsibility for Employers Regarding Health Coverage For 2026, the penalty for not offering coverage at all is $3,340 per full-time employee (after subtracting the first 30). The penalty for offering coverage that’s too expensive or too thin is up to $5,010 per employee who ends up receiving subsidized marketplace coverage.
Once an employer decides to offer coverage, two federal mechanisms effectively require an annual enrollment window. The Employee Retirement Income Security Act of 1974 sets standards for how employer-sponsored health plans must be administered, including disclosure of plan information and a process for employees to make benefit elections.4U.S. Department of Labor. ERISA More directly, most employers use what the IRS calls a Section 125 cafeteria plan so employees can pay their share of premiums with pre-tax dollars. Cafeteria plans require employees to lock in benefit elections before the plan year starts, with mid-year changes allowed only after specific qualifying events.5eCFR. 26 CFR 1.125-4 – Permitted Election Changes That lock-in rule is why HR sends the reminders every fall. Miss the window and you’re stuck with your current elections until next year.
Small employers — those with fewer than 50 full-time equivalent employees — face no federal penalty for skipping health benefits entirely. Many still offer coverage to compete for workers, and those that do follow the same ERISA and Section 125 rules.
Medicare Has Its Own Required Enrollment Periods
Medicare runs on a completely different enrollment calendar from the ACA marketplace, and missing its deadlines can cost you permanently.
The annual Medicare open enrollment period runs October 15 through December 7 each year, with changes taking effect January 1.6Medicare.gov. Open Enrollment During this window, you can switch between Original Medicare and Medicare Advantage, change your Medicare Advantage plan, or join, drop, or switch a Part D prescription drug plan.
Separately, most people first become eligible for Medicare around their 65th birthday. Your Initial Enrollment Period spans seven months — three months before the month you turn 65, your birthday month, and three months after.7Medicare.gov. When Can I Sign Up for Medicare If you delay signing up for Part B without qualifying employer coverage, you’ll pay a late enrollment penalty added to your monthly premium for as long as you have Part B.8Medicare.gov. Medicare and You Handbook 2026 The penalty isn’t a one-time fee. It’s a permanent surcharge, and that’s where most people get tripped up.
Medicaid and CHIP Have No Enrollment Window
Medicaid and the Children’s Health Insurance Program work differently. There is no limited enrollment period. You can apply any time of year and coverage begins as soon as you’re determined eligible. Since 2024, federal law also requires states to provide 12 months of continuous eligibility for children under 19 in both programs, so a child’s coverage cannot be dropped mid-year even if the family’s income fluctuates.9Medicaid.gov. Continuous Eligibility for Medicaid and CHIP Coverage
If your income is low enough to qualify for Medicaid, you don’t need to wait for open enrollment. Apply directly through your state’s Medicaid agency or through HealthCare.gov, which will route you to Medicaid if your income falls below the threshold.
If You Miss the Window
If you miss marketplace open enrollment, you may still be able to enroll through a Special Enrollment Period triggered by a qualifying life event. The most common triggers include losing existing coverage (a job-based plan, a parent’s plan at age 26, or Medicaid), changes in household (marriage, birth, adoption, or divorce that ends spousal coverage), and moving to a new ZIP code or county with different marketplace plans, or moving to the U.S. from abroad.10HealthCare.gov. Getting Health Coverage Outside Open Enrollment
You generally have 60 days from the qualifying event to enroll. For Medicaid or CHIP losses, the window extends to 90 days.11CMS. Understanding Special Enrollment Periods
If no qualifying event applies, you’ll go without ACA-compliant coverage until the next open enrollment, which can mean a gap of up to 10 or 11 months. The federal individual mandate penalty was effectively eliminated starting in 2019, so there is no federal tax for being uninsured.12HealthCare.gov. Exemptions From the Fee for Not Having Coverage A handful of states and the District of Columbia enforce their own individual mandates with financial penalties, which vary. Some are income-based at 2.5% of household income; flat-dollar penalties range from roughly $300 to over $900 per uninsured adult depending on the state and your income.
The bigger risk during a coverage gap is medical. A single emergency room visit or unexpected diagnosis without insurance can produce bills in the tens of thousands of dollars. Short-term plans exist to fill gaps, but they aren’t required to cover pre-existing conditions, often exclude maternity care, prescription drugs, and mental health services, and may cap total benefits as low as $100,000 per coverage period.