About 5p from every copy of Now sold in the UK will go to it.
Girls Aloud member Kimberley Walsh has guest-edited the special issue, where she recounts her experiences of visiting Uganda for Sport Relief.
The English singer visited the African country to see how the money she raised by climbing Mount Kilimanjaro last year for Sports Relief was being used. Walsh is also the guest commentator on Now's 'The Week In Outfits' fashion column.
The issue also carries an interview with comedian David Walliams, who led a celebrity bike ride to raise £1m for Sport Relief.