Edward Cairney (59) spotted the seemingly disc-shaped object on Wednesday afternoon when looking west towards Dundee from the rear window of his home in Arbroath.
He took two pictures of the UFO, between 1.50-2.10pm, before it disappeared from view.
He said: ''It was around sunset and at first I thought it was maybe just the moon shining in a strange way or something like that.''
He added: ''I was standing at the back window looking to the west towards Dundee. There were a couple of passengers planes in the sky and, as the sun set, there was a bit of an air passage towards the west.
''The planes were painted red by the sunshine but this thing wasn't. It was just so weird.''
Mr Cairney, who lives in Strathairlie Avenue, said his son Ross (28) also saw the light as he was driving to Dundee.
''My son was travelling to Dundee and he phoned to say he saw it. He was about 15 miles away at the time and he said it looked like a light in the sky.''
A spokesman for RAF Leuchars confirmed there had been no activity in the area and nothing unusual had appeared on the radar.
After seeing the picture Matt Lyons, chairman of the British UFO Research Association, suggested a number of theories -- including a streak on the camera lens -- but said further analysis would be required.