The airport said on Monday night that the UFO showed up on its radar several times over three hours from 6.30pm to 9.30pm.
Police said in a statement that they were alerted at 6.30pm about a UFO over Bremen and a patrol car also saw the object.
A police helicopter was sent up to try and find the UFO but could not, although the Weser Kurier newspaper reported it could be seen through binoculars from the airport's traffic control tower.
The helicopter search was called off at 8.45pm, according to the police statement.
Officers are now investigating the case on grounds of unauthorized intrusion into controlled air space.
A spokeswoman for German Air Traffic Control (DFS) said: “It had normal lights, red and green, and an approach light on both sides," the Weser Kurier reported.
Eyewitness Dominique Höber told the paper the object was flying at between 100 and 200 metres and passed over his house four times.
“It looked like a plane,” he said. “It had lights but was a lot louder.”
A flight from Frankfurt had to be cancelled and a second flight from Munich was diverted to Hannover.
Another plane coming from Paris had to stay airborne for a while before it could land.
A Bremen police spokesman said. “We still don’t know what it was, but it was there.” Possible explanations are that it was a drone or a balloon-like aircraft.
On Sunday in the town of Zwickau, Saxony, an initially unidentified aircraft landed on a field behind a house.
Hours later it turned out to be a remote-controlled model airship.
Eyewitnesses should contact Bremen Police on 0421 362 3888.