Gary McDermott snapped the glowing red object, with bright flashing lights, after stopping his car to photograph a low-flying helicopter in Plymouth.
The disc-shaped UFO flashed across the sky - just as he was taking the picture - before it disappeared into the night at 9pm on Sunday.
Mr McDermott, who was working night shifts on the city's famous Royal Albert Bridge, said: "I just couldn't believe what I had just
seen.