Under the agreement, O2 customers will be able to buy single track downloads from a archive on their mobiles for a limited promotional price of 99p, or five songs for £4.
The network provider said that once a song has been bought through O2 Active's mobile service, a copy will automatically be sent to an email address of the customer's choice.
The Napster service will give O2 customers using 30 different types of handsets mobile access to the latest chart releases, as well as download options from a 5m-strong music archive. O2 already has a existing download agreement with Napster in Ireland.
Sally Cowdry, marketing director at O2, said: "Working with the right partners at O2 UK is crucial to our success and we have joined forces with a leader in this space to bring our customers a high quality, great value music service with the widest choice of songs available on mobile."