"This morning we experienced intermittent issues with our passenger service computer system, causing some flight delays," the company said in a statement.
"With the system now restored, we are working to get our customers to their destinations as soon as possible," the statement said.
The Chicago Tribune reported that United had frequent computer problems in 2012, when it was merging reservation systems with Continental airlines.
Tuesday's delays provoked some griping on Twitter. "All computers at @united are down. Giant lines everywhere," one post read. "Got resourceful, and managed to produce a boarding pass from home," the customer wrote.