Apple is describing the Complete My Album scheme, as an "additional incentive" for users to download albums, by giving money off to customers who buy albums from its .
Under the new rules, a customer who has bought two tracks will now pay less than the full cost of an album on iTunes if they choose to download an entire album.
Previously, customers would either have to repurchase tracks they already own when downloading an album, or face buying the remaining songs on an individual basis, which costs more.
The decision means consumers will get a better deal from Apple when they use the iTunes Store, which receives around 55% of its revenue from people downloading individual tracks.
Eddy Cue, vice-president of iTunes, said: "The idea here is simple -- once you've bought singles from an album, we'll give you credit for it. This is something that is another example of how digital music is a changer in the way we do things."
Apple said the Complete My Album'service would also be "retroactive" for the next 90 days, allowing iTunes users to buy additional album tracks dating back to when the store launched four years ago.