Morgan Coxhead, 15, who has had the genetic condition since birth, saw the young girl come to a standstill as she became overwhelmed by the crowd during the Great South Run event earlier today.
Attempts by others to rally the girl were failing to push her on before Morgan's quick-thinking saved the day.
Morgan, who used to live in Portsmouth but now lives in Cornwall and runs a charity Just Breathe Cornwall, said: '˜I saw the girl, who looked like she had a disability, and decided to go and help her. She was struggling to finish and seemed a bit overwhelmed by everything.
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'˜I decided to take my dog over as I thought that might help her. Fortunately she loved it and it helped her get over the line. She was very grateful.'
Morgan, meanwhile, finished as one of the front runners in the canine race after registering a time of nine minutes 43 seconds '“ with him raising £500 for his charity.Â
'˜I've had cystic fibrosis since birth but I don't think having my condition or any condition should hold you back,' he added.
His brother Aidan, 18, who has autism, also ran in the 10-mile race.
Mum Vicky said: '˜ I'm really proud of them both. It was lovely what Morgan did with the little girl to help her finish.'