The theft took place in March from the offices of computer software firm NorthgateArinso which provides software to The Pensions Trust.
The data included names, addresses, dates of birth, employers, national insurance numbers, salary details and bank details. It was protected by a password but was not encrypted.
The data was stolen from members of six of the 39 pension schemes administered by The Pensions Trust.
The schemes included social housing, SSHA, independent schools, flexible retirement plan, growth plan (series 1, 2 & 3) and the unified ethical plan.
Spokeswoman Lynda Howe says: “NorthgateArinso has expressed their regret that this theft has occurred and investigations are ongoing.
“I can confirm that The Pensions Trust has now withdrawn access to personal member data from NorthgateArinso and have also instructed them to delete any existing personal member data they hold.
“We are hopeful that this incident will not have any impact on members but as a precaution, we have arranged for them to be protected by CIFAS, the UK’s fraud prevention service, and have set up a members helpline.”