The 737-800 is a stretched version of the 737-700, and supersedes the 737-400. It also filled the gap left by the decision to discontinue the McDonnell Douglas MD-80 and MD-90 following Boeing's merger with McDonnell Douglas. The −800 was launched by Hapag-Loyd Flug (now TIIfly) in 1994 and entered service in 1998. The 737-800 seats 162 passengers in a typical two-class layout, or up to 189 in one class. It competes with the A320. For many airlines in the U.S., the 737-800 replaced aging Boeing 727-200 trijets and MD-80 series and MD-90 aircraft.

The P-8 Poseidon is a 737-800ERX ("Extended Range") that, on June 14, 2004, Boeing's Integrated Defense Systems (now called Boeing Defence, Space & Security) division beat Lockeed Martin in the contest to replace the US Navy's P-3 Orion maritime patrol aircraft.

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