To prove its claim, the LulzSec group published the names, birthdates, addresses, emails, phone numbers and passwords of thousands of people who had entered competitions promoted by Sony.
"From a single injection, we accessed everything," the hacking group said in a statement.
"Why do you put such faith in a company that allows itself to become open to these simple attacks?"
Details of those hacked into were posted on the group's website but they have subsquently been removed.
A Sony spokesman said the company was "looking into these claims" but refused to say anything further.

One person whose details were retrieved was 84-year-old Mary Tanning, from Minnesota, who confirmed the details were correct.
"I don't panic," she said, going on to explain that she was not very well off and was very seldom online. "There's nothing that they can pick out of me."
Another target, Elizabeth Smith from Tucson, Arizona, was angry.
"If this is so, I'm very upset," she said. "I'm very disappointed that Sony would not protect things like that."
Ms Smith said she often entered online sweepstakes, including ones connected with Sony. She said she had not heard anything from Sony itself about the breach.

We have a Playstation 3 section, PS3 News gos in there, surely you know that.