The man released was aged 21. He was no longer suspected of involvement in the August 18 attack that killed two Finnish women and injured eight other people, police said.
The main suspect - a Moroccan identified as Abderrahman Bouanane, 22 - remains in pre-trial detention.
Bouanane, who was shot in the thigh by police responding to the attack, remained in a prison hospital in Hameenlinnaa, north of the capital of Helsinki, Markus Laine of the National Bureau of Investigation told dpa.
The attacks were designated as two cases of murder with terrorist intent and eight cases of attempted murder with terrorist intent.
Bouanane can Tuesday at the earliest contest his pre-trial detention at a court in Turku.
Laine said an international warrant for another suspect was still active, but declined to comment on the identity or suspicions regarding that person.
Earlier this week, two other men held on lesser suspicions of involvement in the attack were also released and no longer considered suspects.
They have cooperated with authorities, Laine added.
Officials standing in a street in the Finnish city of Turku where several people were stabbed. August 18, 2017 file picture
A man suspected of involvement in a stabbing spree that claimed two lives in the centre of the south-western Finnish city of Turku has been freed, leaving only the chief suspect in pre-trial detention, authorities said Friday.
