The quartet, who have had five number ones, have recorded the song Proud for the fundraising extravaganza, which takes place in March.
They follow in the footsteps of artists such as Robbie Williams, McFly and Leona Lewis on previous releases for the biennial charity event.
The JLS track will go on sale as a download from March 18, and then on sale in shops the following day.
The group will be performing a one-off matinee concert for Sport Relief at the O2 Arena on Saturday March 24, with tickets going on sale on Friday morning.
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