The UA971 flight was over the Atlantic Ocean when it was diverted to Shannon Airport in Ireland, landing at about 2:15 p.m. local time.
The passengers disembarked and underwent further screening after a bomb threat was found scribbled in one of the plane's bathrooms.
According to Irish media reports, passengers were providing handwriting samples to the flight crew to see if any matched the writing found in the bathroom.
They will likely spend the night in Shannon before continuing to their final destination.
In a statement, Chicago-based United Airlines said: "United flight UA971 en route to Chicago from Rome, diverted to Shannon following a potential security concern. Additional security screenings will be performed on all customers and baggage, and we will work to get customers on their way to Chicago as quickly as possible."