Simmonds missed out on a medal at the Tokyo Games and admitted afterwards that she was considering walking away from the sport after a decorated career.
The five-time Paralympic S6 swimming gold medallist confirmed the decision in a statement on Thursday afternoon and thanked the public for their support.
“Back home from Tokyo and I’ve finally managed to go through all of the wonderful messages I’ve received from so many good people,” she wrote. “Thank you from the bottom of my heart to everyone who messaged me, I am genuinely overwhelmed.
“I’ve heard it said by others that you know when it is time to call it a day, and having given it a lot of thought, I know that now is the right time for me.”
Simmonds won two gold medals at the 2008 Beijing Olympics, at just 13-years-old, to become a household name and she was one of the faces of the London 2012 Games.
She won a further two golds in the home Paralympics and added her fifth at the 2016 Rio Games.
After finishing fifth in her final race at Tokyo, Simmonds revealed she had gone into the Games thinking it would be her last.
She told Channel 4: “I think for me, yes, this is going to be my last but I will go home and evaluate.
“I’m not just saying that because I’m gutted or anything like that, I knew going into these Games this was going to be the last and I don’t think I could go for another three years. I’m leaving it at the right time, I love it, I’ve absolutely had a wonderful competition and I’ve loved every minute of it.”
While she has retired from competing, she did say she wouldn’t be giving up the sport all together.
“I love swimming and I’ll still be found in the pool, but now I’ll be able to do that without the 4.30am alarm!” she said.
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