The ship, the Baltic Leader, was heading from the north-western French city of Rouen to St Petersburg in Russia with a cargo of new cars.
It is being held at the Channel port of Boulogne.
Russian state media say the vessel is owned by a subsidiary of a bank targeted in recent EU and US sanctions.
A regional French official said such a measure was rare, but called it a sign of firmness.
The US Treasury Department has issued blocking sanctions against the vessel, saying it is owned by a subsidiary of Promsvyazbank, one of the Russian financial institutions hit by sanctions.
French officials said they were checking whether the vessel was linked to Russian financial interests and that the crew were “being co-operative”.
The process could take up to 48 hours, they added.