Indonesia's Lion Air Group has committed to 50 Boeing 737 Max 10s.
Boeing, which puts the deal's list-price value at $6.24 billion, says details will be posted on its orders and deliveries website "once finalised".
Lion Air Group president Edward Sirait describes the 737 Max family as "the perfect combination of single-aisle aircraft", citing their "new-level fuel efficiency, as well as range and capacity capabilities".
Boeing notes that Lion has previously ordered 201 Max jets, and is one of the world's largest 737NG operators.
The airline is the Max 9's launch customer, and its Malaysian subsidiary Malindo Air gave the Max 8 its commercial debut