The bank told a number of customers using claims management firms that they did not hold PPI policies when they did.
A "very small percentage of customers" were given the wrong information when they contacted the bank via a third party, Barclays said, and the bank is "proactively contacting" them to make a correction.
PPI was offered to customers to insure the repayment of borrowings to protect against accident, illness, unemployment or death, however some policies were mis-sold.
A Barclays spokesperson said: “Following a recent review, Barclays will be putting things right for customers who were given the wrong information when they contacted Barclays (via third party) to find out whether they had PPI on their account.
"We would like to apologise to those customers impacted. Our complaints service and customer direct PPI checking services are not affected by this matter.”