The commitment, announced at the Singapore Airshow, is for eight next-generation 737-800s and seven 737 MAX 8s, Nok Air and Boeing said.
"This commitment is a major step in our growth strategy," Nok Air chief executive Patee Sarasin said in a statement.
"The 737 is the backbone of our fleet and will continue to be in the future."
Speaking at a news conference, Mr Patee said that his airline would order more planes in the future but would take a more cautious approach compared to other low-cost carriers.
"There are going to be more aircraft coming in," he said.
"We have a lot of competitors who love to buy 200 or 300 aircraft," he said, citing Tony Fernandes, the chief executive of Malaysian budget carrier AirAsia.
"He bought so many aircraft, and currently he doesn't know where to fly to. Our strategy will not be the same as Tony Fernandes," he added.
"We actually plan our strategy in terms of destinations we fly to first, before we order our aircraft."
Shares of Nok (NOK) closed the morning session on the Stock Exchange of Thailand at 18.20 baht, unchanged, in trade worth 5.5 million baht.