The Florida-based airline said Tuesday's giveaway was to thank its customers on the first anniversary of the terror attacks.
All 13,000 seats available on the low-fare airline were taken shortly before 3 p.m. EDT.
"The response was even far faster than we though it would be," said Ned Homfeld, Spirit's chairman. Spirit made the offer after major carriers like American Airlines and United reduced flights because of slow bookings for the anniversary of the terrorist attacks on New York and Washington.
Spirit passengers already holding tickets for travel on Sept. 11 will receive vouchers good for a future ticket. Those with return flights on other dates will pay half the roundtrip fare.
The Holiday Inn Family Suites resort near Walt Disney World in Orlando said it would match the airline's free travel offer by giving away hundreds of two-bedroom suites for the anniversary.
"Only 250 of the resorts 800 suites were reserved for Sept. 11 so 500 are still available," said spokeswoman Julie Montgomery. "The phones have been ringing off the hook since. We're planning a big, huge party."
Customers previously booked on Sept. 11 will receive vouchers for a free one-night stay. Travelers can contact the resort at 800-374-4935 or visit hifamilysuites.com on the Internet.
"I applaud Spirit Airlines and frankly wish we'd acted first, but we should all be doing everything we can to encourage Americans to travel and to thank those who do," said managing partner Terry Whaples.
He challenged the other 2,100 Travel Industry of America members to match the free travel offer.
"From the moment the terrorist attacks brought tourism to its knees, my partners and I have been saying that we can't let the terrorists take away our freedom to travel, and while all of us will pause that day to remember, we must move on," Whaples said.