An employee at the Finnish Tax Administration viewed taxpayers' information that was not related to the worker's duties, the agency announced in a press release on Wednesday.
The tax authority said that so far, it had not found evidence suggesting that the information viewed by the unauthorised employee had been shared or otherwise mishandled.
The agency said it became aware of the matter through its own monitoring.
It said a report has been filed with police regarding the incident, and that it has contacted the Data Protection Ombudsman about the matter.
The agency also said the employee in question no longer has access to Tax Administration equipment or computer systems.
The Tax Administration said it regretted the incident.
"The Tax Administration has taken immediate action and is contacting affected customers as soon as possible," the press release read.
The agency emphasised that in this case, it will not contact customers by phone, text message or email in order to request their personal information.
It also noted that customers do not need to contact the agency about the matter.
Going forward, communications about the matter will be handled by police, the agency said.