Participation from young, healthy people will be crucial to keeping premiums low, but this population is often the least likely to buy health insurance.
"Some of you may feel like you're too healthy to need health insurance, or it's just too expensive," Sebelius said.
"But we never know when we'll get into a car accident, or when we'll need to make that unexpected trip to the ER. ... Without insurance, we have to pay for all these things out of our own pockets."
Sebelius's PSAs are part of a larger effort to sell the law to the public using as many prominent voices as possible.
Several celebrities, such as Katy Perry, have tweeted support for ObamaCare, though it's unclear if any stars will appear in direct advertising for the law.
The marketplaces open for enrollment Oct. 1.