The man, who has not been named, was arrested on Monday (July 22) by officers from the National Crime Agency's Cyber Crime Unit.
He was held on suspicion of fraud offences under the Computer Misuse Act, but has since been released under investigation, the NCA said.
Student records, phone numbers and ID documents were accessed in the hack, which came to light on Friday, July 19.
"We are aware that fraudulent invoices are being sent to some undergraduate applicants. We have alerted applicants to be aware of any suspicious approaches," the university said.
The National Crime Agency said it is continuing to work with the university to find out how the data breach occurred.