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".
The managing director of the ship's owner, Thorco Shipping, expressed surprise. Thomas Mikkelsen told AFP news agency from Denmark that he was unaware of the matter.