The deal is subject to financing, which is now being worked on with the banks and Finnvera, Finland's export credit agency.
No price was reported but this year's Mein Schiff 1 is listed at the euro equivalent of $515m in Seatrade's orderbook.
'We are very thankful that TUI Cruises is entrusting us with building another ship for their fleet of modern and beautiful Mein Schiffs,' Meyer Turku CEO Jan Meyer said.
The order will give further work to Turku shipyard and the Finnish maritime cluster. 'This is what we are preparing for with our recruitments and investments,' Meyer noted.
Mein Schiff 1 is scheduled for delivery this spring, followed by Mein Schiff 2 in spring 2019.