The iOS app, made by Appshen and launched last year, aims to educate kids on why it is important to avoid junk food and eat only healthy meals and snacks.
Michael Johnson's Fun Run is an arcade-style mobile game that involves players using their fingers to sprint, with fast food slowing them down and healthy food speeding them up.
Strong performances are rewarded with bronze, silver and gold medals, while the app also serves up various food-related facts.
The app was previously priced at 69p, but is now being offered on the iTunes App Store for free.
"We need more children to understand the importance of a healthy lifestyle, and exercise and a good diet are key to achieving this," said four-time Olympic Gold medallist Johnson.
He added that the app "will hopefully get kids interested in running and also teach them about a balanced diet in a fun way".
Alongside Fun Run, Appshen also teamed up with Johnson to launch the Motiv8 app last year, offering time-splicing technology that enables users to have the world-class athlete as their "personal wingman".
Later this month, the company will release a new mobile game app with another famous Olympian, Sir Steve Redgrave.