The fares advertised by
But when would-be fliers tried to book the fares, they found no seats available on those routes.
The regulation requires airlines to display the full cost of all fares, including taxes and fees. Airlines that advertise fares for specific routes must also offer a "reasonable numbers of seats" for those flights.
In addition to the fine issued Thursday, the DOT ordered the airline to pay a $100,000 penalty that had been suspended after a previous violation of the same rule.
Southwest Airlines said the latest violation was the result of an error in a television ad. The airline said it never intended to offer the $59 fare for flights to New York, Los Angeles and Chicago.
"As soon as we became aware of our mistake, we pulled all incorrect advertisements off the air," the airline said in a statement.
In the previous violation, Southwest advertised $66 one-way fares from Dallas Love Field to Branson, Mo., between March 1, 2013, and March 21, 2013. However, the DOT found that there were no seats available at the advertised price during the sale period.