An ICO investigation found that the attack could have been prevented if the software had been up-to-date, while technical developments also meant passwords were not secure.
David Smith, Deputy Commissioner and Director of Data Protection, said:
"If you are responsible for so many payment card details and log-in details then keeping that personal data secure has to be your priority. In this case that just didn't happen, and when the database was targeted - albeit in a determined criminal attack - the security measures in place were simply not good enough.
"There's no disguising that this is a business that should have known better. It is a company that trades on its technical expertise, and there's no doubt in my mind that they had access to both the technical knowledge and the resources to keep this information safe.
"The penalty we've issued today is clearly substantial, but we make no apologies for that. The case is one of the most serious ever reported to us. It directly affected a huge number of consumers, and at the very least put them at risk of identity theft.
Following the breach, Sony has rebuilt its Network Platform to ensure that the personal information it processes is kept secure.
The ICO is the UK's independent public body set up to uphold information rights in the public interest, promoting openness by public bodies and data privacy for individuals.