Warner Music Group is making its music available for US downloads from Amazon in MP3 format without copy protection.
Warner had been holding out against using the format because MP3 tracks are easier to share between users and may be freely burned onto CDs.
Amazon.com's download store is a major US competitor to Apple's iTunes, which uses Digital Rights Management (DRM) to restrict the use of some of its tracks.
Warner's artists include Led Zeppelin, Aretha Franklin and Sean Paul.
Sony BMG is now the only major recording group not signed up with Amazon.com's download service, which is only available to US customers at present.
"By removing a barrier to the sale and enjoyment of audio downloads, we bring an energy-sapping debate to a close," Warner Music chief executive Edgar Bronfman said in an e-mail to Warner employees.
Amazon launched its US download store in September after reaching agreements to sell unprotected tracks from Universal Music Group and EMI.