Torstein Hagen's Viking Ocean Cruises has signed up for two more cruiseships at Fincantieri in Italy.
The vessels come with options for two additional units and will be delivered in 2021 and 2022.
Fincantieri said they are sisters to the six already ordered at the yard.
Final contracts will become effective when financing is fixed.
Each ship is 47,800 gt and accommodates 930 people in 465 cabins.
The first in the series, Viking Star, was delivered in 2015. The fourth will be handed over later this year, with number five due in 2018 and number six the year after.
These will make Viking the largest small-ship ocean cruise line.
Viking added the fifth and sixth ships in 2015.
The initial four-vessel order was priced at EUR 300m ($319m) each.
Fincantieri CEO Giuseppe Bono said: “The extraordinary moment of the cruise sector is continuing, and with this announcement Fincantieri reaffirms its worldwide leadership in the segment.
"As worldwide river cruise leader, Viking decided to enter the ocean cruise segment through a start-up together with Fincantieri as shipbuilder, achieving a huge success."
Viking chairman Hagen said: “Our guests and the industry have spoken, and they love the ocean ships that we have built with Fincantieri.
“We set out to bring the destination back to cruising, and the cruising public has responded. These additional ships are needed to meet the demand we are experiencing."
