The 23,200-gt Dorset (ex-Longstone, built 2003) was acquired on Thursday from Belgian owner Shiplux VIII.
It was previously operated by a joint venture including UK James Fisher.
The ship will be renamed Finnmerchant and operate from 19 January between Hanko and Rostock.
Finnlines also agreed to buy two ro-ros last Friday for North Sea operations by the end of 2015, but did not disclose the owner.
But
