This summer I didn't sign up for my employer plan because premiums were too high. I just found out I won't qualify for premium tax credits with a marketplace plan because an employer plan is available to me. Will I have to pay a penalty?

Individuals with no coverage | Individual Responsibility Requirement (Individual Mandate)

Yes, unless you qualify for an exemption. See this FAQ for a list of exemptions.

Individuals with no coverage
Individuals with coverage
Coverage for small employers
Post enrollment issues