Police in Sydney, who interviewed all 246 passengers, later confirmed the threat was a hoax, said local commander Peter O'Brien.
UA Flight 840, carrying 246 passengers, turned around "when an object which raised some security suspicions was found on board," the company said in a brief statement.
"As a precaution, the captain immediately returned to Sydney landing without incident at 5:50 p.m. (3:50 a.m. EDT). Further investigations will be carried out," United Airlines said in its brief statement.
The hoax came just days after a purported al-Qaida affiliate in Europe, the Tawhid Islamic Group, warned it would turn Australia into "pools of blood" if Canberra doesn't withdraw its troops from Iraq. Australian has nearly 900 military personnel in and around Iraq.
Transport Minister John Anderson told Australian Broadcasting Corp. radio the "object" discovered was a note carrying a bomb threat. He said an investigation would immediately be launched.
Australian media reported the note was written on an air sickness bag found in or near one of the aircraft's toilets.
Anderson said the note carried "some words that implied that there might have been a bomb on board." He said he had not heard of any other threat directed at the flight.
Even so, flights in and out of Sydney, Australia's busiest airport, were briefly halted or diverted during what the government treated as a full-scale emergency, Anderson said.
"The first point to make is everyone is safe and flights in Australia are now resuming," he added. "Things are returning to normal."
Anderson said the plane was being searched but no bomb was immediately found. Later he told ABC television he was "pretty sure it was a hoax."
O'Brien said all passengers were interviewed and released after the Boeing 747 landed and taxied to a remote spot at the airport and they were bused back to the terminal.
The plane, which took off at about 1 a.m. EDT was 90 minutes into the flight when the pilot made the decision to return to Sydney, the airline statement said.
The flight was rescheduled to fly to Los Angeles late Wednesday morning.
By Mike Corder