US owner Lindblad Expeditions Holdings has returned to Norwegian yard Ulstein for a second cruiseship.
The company said the move would expand its National Geographic polar fleet to four ships.
Ulstein is building the third of these for delivery in 2020.
The latest addition to the orderbook is scheduled for completion in the third quarter of 2021.
It will be "fully stabilised with the highest ice class (PC5 Category A) of any purpose-built passenger vessel," the company said.
CEO Sven Lindblad added: “These two ships will represent a whole new level of capability in expedition cruising, in terms of the level of comfort they provide guests, the regions and itineraries they are able to travel to and their ability to act as platforms for science in some of the most remote and pristine parts of the world.”
Like the sistership, the new unit accommodate 126 passengers.
In 2017, the original order at Ulstein came with two optional ships.
The first newbuilding cost a reported NOK 1.1bn (then $134.6m).
A fourth ship in the polar series was announced last year, but no yard was given for the deal.
