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.
MORE HBJ HEADLINES
Everett's ATS CEO named HBJ Executive of the Year Lawmakers urge Boeing not to sell aircraft to Iran 3:32 p.m. American Airlines dumps Zodiac, goes with B/E Aerospace U.S. businesses add jobs at slowest pace in 3 years Aircraft supplier plans job cuts, facility closings 3:40 p.m. 500-euro bill, beloved by money launderers, is discontinued