Police are questioning the crew of the MS Thor Liberty after what were described as 69 Patriot anti-missile missiles were found aboard.
Interior Minister Paivi Rasanen said the missiles were marked “fireworks”.
The MS Thor Liberty had docked in the Finnish port of Kotka after leaving Germany last week.
Dock workers became suspicious after finding explosives poorly stored on open pallets, and the missiles were then found in containers marked “fireworks”.
Police did not confirm Finnish media reports that the ship had also been scheduled to stop in South Korea, Reuters news agency reports.
Patriot missiles, designed by the US company Raytheon, are supplied to “US and allied forces”, according to the company’s website. South Korea is among states which deploy them.