The aerospace giant said Southwest is the first customer to finalize an order for the 737 MAX, which is expected to enter service in 2017.
The firm order is the largest in Boeing's history both in terms of dollar value and the number of airplanes, the company said.
Southwest also ordered 58 Next-Generation 737s, it said.
Boeing shares closed at $70.90 Monday on the New York Stock Exchange.
(Reporting by Bijoy Koyitty in Bangalore; Editing by Saumyadeb Chakrabarty)