Boeing has disclosed the decision by the leisure carrier, which is jointly owned by Lufthansa and Turkish Airlines.
Two years ago, SunExpress placed 40 firm orders for current-generation and re-engined 737s, taking 10 options.
The firm orders included 25 737-800s, with the balance covering Max 8s.
SunExpress's German arm says all the aircraft are being delivered to its Turkish sibling.
The airline took delivery of five 737-800s in 2015 and is set to receive seven more this year.
Deliveries under the original order agreement will run through 2021.