The Finnish parliament on Monday said that police were investigating the hack of a number of email accounts linked to lawmakers and employees.
The attack took place during recent months and was discovered by parliament's technical surveillance systems.
The National Bureau of Investigation (NBI) has launched a probe into the incident with support from parliament.
It is not known who was responsible for the hack.
The speaker of the parliament, Anu Vehvilainen, said the attack was "a serious threat against our democracy and Finnish society."
"To strengthen cyber security, we need our own national measures as well as active cooperation both within the EU and internationally," she added.
The Finnish Parliament had no further comment due to the ongoing probe.
"This is not accidental," said Detective Superintendent Tero Muurman with the NBI, adding that as the investigation was ongoing it was not possible to say how many people were affected.
The case was "exceptional," he added.
The NBI was cooperating with other authorities in Finland, including the security police and international partners.
Neighbouring Norway's parliament reported a similar hack earlier this year which the country's police security service (PST) has blamed on hackers linked to Russia's military intelligence.