The Chicago-based airline raised all of its domestic fares by $2 to $5 each way, according to Rick Seaney, founder of FareCompare.com, which tracks airfares.
Jamie Baker, an airline analyst with J.P. Morgan, says that the carrier raised short-haul fares for flights below 500 miles by $2, fares for flights between 500 and 1,000 miles by $3, and fares for long-haul domestic routes by $5.
Seaney says more than 1 million United fares increased.
As of Thursday morning, no other airline had matched the increase. "American is still evaluating," said Matt Miller, a spokesman for American Airlines.
Typically, Southwest Airlines and other low-cost carriers have to match the increase in order for it to stick.
"Participation by Southwest is required to ensure widespread industry success, in our view," Baker wrote in an email. "We are cautiously optimistic."
Southwest last made a broad-based effort to raise fares on Oct. 12. On Nov. 30, the airline raised fares just in the Florida market.
Last year, airlines made 15 attempts to raise fares, according to FareCompare.com. Only seven were successful.
Baker said that "it often requires several attempts to reach industry consensus, and no individual failure should be viewed as a 'lack of pricing power.'"
"We ascribe a reasonably high likelihood of industry success to United's latest effort, though we would anticipate roll-backs over the weekend should Southwest remain silent," Baker said.