Northwest Airlines, Inc. (often abbreviated NWA), a wholly-owned subsidiary of Delta Air Lines, Inc., is a major United States airline headquartered in Eagan, Minnesota, near Minneapolis-St. Paul International Airport in the United States. Northwest has three major hubs in the United States: Detroit Metropolitan Wayne County Airport, Minneapolis-Saint Paul International Airport, and Memphis International Airport. Northwest also operates flights from a hub in Asia at Narita International Airport near Tokyo and also operates transatlantic flights in cooperation with partner KLM from Schiphol Airport in Amsterdam. Additionally, it maintains focus city operations at Seattle-Tacoma International Airport.

As of 2006 Northwest was the world's sixth largest airline in terms of domestic and international scheduled passenger miles flown and the U.S.'s sixth largest airline in terms of domestic passenger miles flown. In addition to operating one of the largest domestic route networks in the U.S., Northwest carries more passengers across the Pacific Ocean (5.1 million in 2004) than any other U.S. carrier, and carries more domestic air cargo than any other American passenger airline. It is the only U.S. combination carrier (passenger and cargo service) operating dedicated Boeing 747 freighters. The airline, along with its then-parent company, Northwest Airlines Corporation and subsidiaries, operated under Chapter 11 bankruptcy protection which, in the United States, allows continued operation during the reorganization effort, not cessation of flights as in the case in some countries. Northwest emerged from bankruptcy protection on May 31, 2007.

Northwest Airlines' regional flights are operated under the name Northwest Airlink by Mesaba Airlines, Pinnacle Airlines, and Compass Airlines. Northwest Airlines is currently a minority owner of Midwest Airlines, holding a 40% stake in the company.Northwest Airlines Becomes Minority Owner of Midwest Airlines (Midwest Airlines Official Press Release: August 17, 2007) Its frequent flyer program is called WorldPerks. Northwest Airlines' tagline is "Now you're flying smart."

On April 14, 2008, Northwest announced it would be merging with Delta Air Lines. The merger closed on October 29, 2008, making Delta the largest airline in the world. However, Northwest will continue to operate as an independent carrier (as a Delta Air Lines subsidiary) for several months until the operating certificates and other factors are combined.

In February 2009, the airline began consolidating gates and tickets counters at airports where both Delta and Northwest Airlines operate. The rebranding will include the changing of "Northwest" signs to "Delta" signs. The integration is expected to continue throughout this year and into early 2010. The airline's hubs in Detroit, Minneapolis/St. Paul, and Memphis were rebranded as of March 31, 2009.