Boeing said Thursday that the pending order is valued at $6.3 billion at current list prices.
The deal includes 40 737 Max 8 planes and 25 737 Max 7 aircraft. Deliveries are expected to start in September 2017.
The 737 Max will have CFM International LEAP-1B engines, and is expected to reduce fuel burn and CO2 emissions by 13 percent as compared with the most fuel-efficient single-aisle aircraft currently available.
WestJet Airlines Ltd.'s current fleet includes 103 Boeing next-generation 737 and four Bombardier Q400 next-generation aircraft.
Boeing Co. currently has 737 Max orders from five other North American airlines.