The DOT said the Dallas-based carrier offered $59 fares to New York, Los Angeles and Chicago last October but there were no $59 seats on flights to those cities during the fare sale travel dates, the government found.
"Consumers have rights, and DOT will continue to take enforcement action against carriers and ticket agents when our price advertising rules are violated," said Transportation Secretary Anthony Foxx in a statement.
This is the second advertising violation for Southwest. In early 2013, Southwest offered nonstop fares for $100 or less on Valentine's Day but did not include a reasonable number of seats on the routes marketed in the fare sale.
Since Southwest violated the cease-and-desist part of the previous violation, the DOT said Southwest was required to pay an additional $100,000.
Southwest said the audio portion of the TV advertisements in the October 13 fare sale incorrectly stated that New York, Los Angeles and Chicago were part of the sale.
"As soon as we became aware of our mistake, we pulled all incorrect advertisements off the air," Southwest spokesman Brad Hawkins said. "Once we discovered the error, we did honor the $59 fare for those consumers who called reservations requesting the $59 price point for those three markets."