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.
"It must have been a UFO - and I cannot believe I am saying that because I don't believe in them usually. I am always sceptical.
"But this was definitely not a normal aircraft. It was red, the shape they say UFO aircraft is, and had two bright lights coming out of it.
