Delivery of the planes, which offer 15 to 20 percent better fuel consumption per seat and have a range of 3,200 nautical miles, will run from 2013 to 2018. Delta said the aircraft had committed long-term financing.
High oil prices are spurring airlines to order fuel-saving planes. Last month, AMR Corp's American Airlines split a record order for 460 single-aisle planes between Boeing and EADS unit Airbus, breaking off its exclusive relationship with Boeing.
Delta sent a request to plane makers last year for proposals to deliver up to 200 planes, with options for 200 more, to replace aging narrowbody models.
The airline had said it was talking to Europe's Airbus, Brazil's Embraer and Canada's Bombardier Inc as well as Boeing. Chief Executive Richard Anderson said in Delta's statement on Thursday that "several very competitive proposals" were received.
The Boeing 737-900ER is the newest member of the Next-Generation 737 airplane family. The planes will be equipped with engines made by CFM International, a joint venture between General Electric Co and France's Snecma.
Boeing, based in Chicago, plans to update its bestselling 737 with a more fuel-efficient engine.
Atlanta-based Delta will starting taking deliveries in two years, with 12 planes expected in 2013, 19 aircraft yearly from 2014 through 2017, and the remaining 12 planes in 2018.
Delta, the world's second-biggest airline behind United Continental Holdings, has invested in new airplane seats and better food offerings to boost revenue while retiring least-efficient planes and reducing flying in areas where revenue is not keeping up with rising fuel costs.
It said the 737 planes it is ordering will have lower unit costs than the older Boeing 757 and 767 and Airbus A320 models they will replace. The new planes will be Delta's first with Boeing's new "Sky Interior," which includes expanded carry-on baggage space and an LED lighting system.
Delta has a fleet of more than 700 planes.
Delta shares were down 4.4 percent at $6.93 in morning trading and Boeing shares were down 1 percent at $61.13.