The airlines' move came as the price for jet fuel hit a record $3.03 per gallon this week.
The airfare increase is the largest single domestic fare hike since the Sept. 11, 2001, terror attacks, The San Francisco Chronicle reported Saturday.
"Three-dollar-plus jet fuel does not make cheap fares economically viable," said Henry Harteveldt, an airline analyst at Forrester Research in San Francisco.
United raised its fares Thursday night; Continental followed Friday.
"The cost of fuel continues to rise," said Robin Urbanski, a spokeswoman for United Airlines. "We must be able to pass along costs just like any other businesses do."
Every $1 increase in the cost of a barrel of crude oil means almost $470 million in additional expense to the U.S. airline industry, Elizabeth Merida, a manager at the Air Transport Association, told the newspaper.