Simmonds swam a fantastic final 50m and said afterwards that the win was for everyone who willed her to victory.
"I just went out there and gave it my all," Simmonds said. "I'm exhausted, I can't believe I did that. It was so tough.
"I saw [Victoria Arlen] on the last 100m and I was like 'I've got to put my head down and do it for everyone who has supported me'.
"My coach said it was going to be won in a 5.19. I didn't believe him but I've just beaten my personal best by six seconds."
Simmonds's gold was GB's ninth so far at the Paralympics.