National Bureau of Investigation (NBI) officials in Oulu, northern Finland, say they are holding a middle-aged Somali national who is suspected of supporting and financing terrorist activities abroad.
According to the NBI, the man is suspected of having financed and promoted the militant Islamist group al-Shabaab's operations in Somalia.
The suspect arrived in Finland last September, according to the NBI.
The preliminary investigation was sparked by the deputy prosecutor general. The NBI says it is cooperating closely with the Finnish Security Intelligence Service (Supo) on the case.
In March, four people were acquitted of financing terror in Finland's first terrorism case. They were accused of sending money to al-Shabaab in Somalia and Kenya. One was also charged with recruitment and preparation for a terrorism-related crime related to his brother and nephew.