The two identical ships will be built by Fincantieri - the largest the Italian shipyard has ever built.
They are due for delivery in 2024 and 2026 respectively.
The order will bring Tui Cruises’ Mein Schiff fleet up to nine ships by 2026.
It is the latest step in the line’s “sustainable growth” strategy as part of its joint venture with Royal Caribbean Cruises Ltd.
Wybcke Meier, Tui Cruises chief executive, said: “We are responding to the wishes of our guests and creating offerings that will also inspire new cruise passengers to spend a holiday with us.
“As with the other ships in the fleet, the passenger/space ratio remains generous - further details of the ships will be announced at a later date.”
Giuseppe Bono, Fincantieri chief executive, added the two ships would be largest evert built in Italy, demonstrating the shipyard’s ability to innovate and create cutting-edge products that are also environmentally friendly.
The ships are LNG powered and consume around 30% less fuel than ships of comparable size.
They will come with exhaust treatment systems that cut sulphur emissions 99%, nitrogen oxide 75% and particulates 60%.