Seabourn Cruises said it signed a letter of intent with Fincantieri to build a fourth copy of the 450-passenger Odyssey-class vessels in its fleet.
The line had said previously that it expected to order such a ship by the end of this year.
"We are pleased to be moving forward," said Seabourn President Richard Meadows.
The new ship will replace capacity that is leaving Seabourn. Its original three ships that carry 208 passengers each are being transferred to Windstar Cruises, starting next year.
Seabourn said it expects the new ship to be delivered in the second half of 2016. Financial terms of the deal weren't disclosed.
This will be the first ship that Fincantieri builds for Seabourn. The three Odyssey-class ships were delivered by the T. Mariotti yard in Genoa.