The first of the ships will be delivered in 2023. The vessels will weigh around 64,000 gross tonnes and will feature 500 cabins.
Three ships will come into service each year over the next three years.
They will have the latest environmental technology and other advanced maritime solutions.
In addition to an innovative design, the vessels will feature ground-breaking options to ensure guest comfort.
MSC Cruises executive chairman Pierfrancesco Vago said: “It is off the back of the great success of our ship-within-a-ship luxury concept that our guests asked us to enter the ultra-luxury segment as a natural evolution of the MSC Yacht Club.
“These ships will be able to offer unique itineraries, thanks to their size, and the guest services will be taken to another level, including our personalised MSC Yacht Club butler service, available 24/7.
“I am especially pleased to be able to further extend our partnership with Fincantieri through this new order. The award-winning Seaside and Seaside-Evo class have already been recognised as ground-breaking and innovative designs.
“We are now introducing another new class, which will establish a new standard of ultra-luxury at sea with ships that will showcase the quality and highest standards that are associated with Made in Italy.”
Fincantieri CEO Giuseppe Bono said: “We are extremely satisfied that, to launch a new class of ships in the luxury segment, MSC Cruises has recognised that our design will help them create a new generation of ships that will be cutting-edge both in terms of safety and technological content, and with particular attention paid to passenger comfort.”
With the latest order, Fincantieri has 53 cruise ships in its order book.