The lift comes after a tweet from officials at about 5 p.m. PT, which stated there was an "IT problem." The airline did not give any more details, and it was not immediately clear how many flights were affected.
United spokesperson Maddie King issued a statement saying crews were working to fix the issue.
"We have issued a ground stop for all domestic mainline flights due to an IT issue. We are working as quickly as possible to resolve this issue and get out customers to their final destinations. We apologize for the inconvenience to our customers."
United Airlines and United Express, based in Chicago, operate more than 4,500 flights a day to 339 airports across five continents.
The Associated Press contributed to this report.