Short answer: On the federal Marketplace (HealthCare.gov), Open Enrollment runs November 1 to December 15 starting with plan year 2027; earlier years ran into January. Outside it, you generally need a Special Enrollment Period.
Open Enrollment is the once-a-year period when anyone can buy, renew, or switch an individual Marketplace plan without needing a qualifying life event. Under the 2025 Marketplace Integrity and Affordability final rule, the federally facilitated Marketplace shortened its window to November 1 through December 15 starting with plan year 2027, bringing it closer to typical employer open enrollment. In prior years it ran from November 1 through January 15.
States with their own exchanges may offer somewhat longer windows, but federal rules now cap every Marketplace Open Enrollment at nine weeks, beginning no later than November 1 and ending no later than December 31. If you miss Open Enrollment, you can usually enroll only after a qualifying event, such as losing other coverage, getting married, or having a baby, which opens a Special Enrollment Period.