Southwest flew out of the now-closed Stapleton International Airport in Denver until 1986, when it left because of high costs. Southwest held off for another decade after DIA opened in 1995, again citing costs.
But the new airport's fees have since declined, luring the airline back.
Southwest's first two flights from DIA were bound for Chicago and Las Vegas, and later flights were scheduled to leave for Phoenix. The Dallas-based airline planned 13 daily flights initially.
At a news conference, Southwest CEO Gary Kelly said the airline will add seven daily flights and two destinations — Baltimore and Salt Lake City — in March.
"We are here. We are here to stay," Kelly said.
Southwest will compete head-to-head with Denver-based Frontier Airlines, a low-fare carrier that also serves Las Vegas, Chicago and Phoenix among its nearly 50 U.S. destinations.
Joan Zack of Brush said as she waited to board a Southwest flight to Phoenix that Southwest's presence could help lower fares out of Denver, where Frontier, United Airlines and United subsidiary Ted handle about 75 percent of passenger traffic combined.
Frontier spokesman Joe Hodas said he did not think Southwest's intention was to put the Denver airline out of business.
"I think their intent is to grow their company as our intent is to grow our company," he said.
Hodas said Frontier and Southwest are matching each other's fares.
He said Frontier's reservations have registered a double-digit increase since Southwest announced its Denver service, but he declined to be more specific.
United, whose parent UAL Corp. expects to emerge from bankruptcy this year, has a hub in Denver.
Shares of Southwest Airlines Inc. fell 17 cents to $16.26 in midday trading on the New York Stock Exchange, while Frontier Airlines Inc. (FRNT) lost 51 cents, or 5.5 percent, to $8.73 on the Nasdaq Stock Market.