In some cases, the address, telephone numbers or other personal information related to the complainants was put on the website in November 2019.
The information was published in a response to a Freedom of Information Act request, which related to the number and nature of new complaints made against the FCA and handled by the complaints team between 2 January 2018 and 17 July 2019.
The watchdog said in most instances, only the name of the person making the complaint was made public.
But the FCA is now contacting those who had further details published to apologise and advise on the extent of information made available.
No financial, payment card, passport or other identity information were included.
In a statement issued today the watchdog said it had only recently been made aware of the data breach.
The regulator added: “As soon as we became aware of this, we removed the relevant data from our website.
“We have undertaken a full review to identify the extent of any information that may have been accessible.
“Our primary concern is to ensure the protection and safeguarding of individuals who may be identifiable from the data…
“We have taken immediate action to ensure this cannot happen again. We have referred the matter to the Information Commissioner’s Office.”