IPods are designed to have a "lock" that means only songs bought on Apple's iTunes can be downloaded on to the portable music player. With iPod the dominant music player in the market, it has helped iTunes to become a massive success for Apple.
In a statement, the company said that it was "stunned" at the move by RealNetworks, and warned people that the technology may not be very reliable.
Apple also said it would be investigating the legal implications of the move, accusing RealNetworks of adopting the "tactics and ethics of a hacker".
RealNetworks has defended the technology, which is known as RealPlayer Harmony.
It said: "Consumers, and not Apple, should be the ones choosing what music goes on their iPod."
Apple has warned that it may thwart Harmony by tweaking code used on iPod.
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