Police on Saturday said they had apprehended Vastaamo data breach suspect Aleksanteri Kivimäki.
Police took him into custody at a private apartment in central Helsinki.
Helsinki police said they tracked Kivimäki down through their own investigations. The arrest was calm, according to the police.
Unknown location
Earlier this week the authorities were unsure of Kivimäki's whereabouts.
On 19 February Western Uusimaa District Court issued a warrant for Kivimäki as he had failed to report to the police.
The prosecutors are seeking a seven-year prison sentence for Kivimäki for crimes against more than 21,000 victims in Finland's largest-ever security breach. He is suspected of hacking a patient record database belonging to the psychotherapy centre Vastaamo, aiming to blackmail tens of thousands of victims.