Mainline flights between major hubs were grounded for over an hour before engineers resolved the issue, AFP reported.
“We are working with customers to get them to their destinations after a technology disruption on Wednesday evening,” the airline said in a statement.
“The underlying technology issue has been resolved, and, while we expect residual delays, our team is working to restore our normal operations.”
The Federal Aviation Authority issued ground stops at several major airports.
The orders affected airports in Denver, Newark, Houston, and Chicago, applying only to United, according to the FAA’s website.
The outage marks the latest disruption in America’s aviation sector.
Last month, Alaska Airlines experienced an IT problem that grounded flights for hours.
Earlier this year, air traffic control failures at a Newark-area airport further eroded passenger confidence.
In January, a mid-air collision near Washington’s Reagan National Airport involving a passenger jet and a military helicopter resulted in dozens of fatalities.