"Dear Lady Gaga," begins the letter by the 18-year-old self-described hacker identified only as Deniz A which was published in the daily Bild .
"I am ashamed of what I have done. I did not think about the consequences."
Prosecutors in the western city of Duisburg have opened a probe against Deniz A. for allegedly trying to sell brand new music by stars including Justin Timberlake, Kesha, Kelly Clarkson as well as Lady Gaga on the Internet.
Deniz A., who reportedly called himself DJ Stolen, told Bild he was "unbelievably sorry".
"I realise that my ambition was a big mistake," he said. "I will apologise to everyone I caused problems for with my hacking."
Duisburg state prosecutor Rolf Haferkamp said the investigation was well underway.
"We are pulling together what we have learned," he told Bild. "They already fill two fat binders. If nothing else happens we could wrap up the probe soon."
Deniz A.'s lawyer Burkhard Benecken said the teenager had been intoxicated by the power of accessing celebrities' personal data.
"My client felt like he was in a movie," he said. "It is almost unbelievable that such a boy could hack into the accounts of superstars. He knows he made a mistake."
Prosecutors say Deniz A. and a 23-year-old accomplice used Trojan horse programmes to penetrate the computers of people close to the artists such as managers.
They also allegedly published an explicit private photo of Kesha on their website.
But Haverkamp has told the German press he thinks it is unlikely they managed to access data directly from the celebrities themselves.