The Northwestern Hawaiian Islands or Leeward Hawaiian Islands are a series of islands and atolls in the Hawaiian island chain located northwest of the islands of Kauai and Niʻihau. Politically, they are all part of Honolulu County in the U.S. state of Hawaii, except Midway Atoll, which is a territory distinct from the State of Hawaii, and grouped as one of the United States Minor Outlying Islands. The United States Census Bureau defines this area, except Midway, as Census Tract 114.98 of Honolulu County. Its total land area is 3.1075 square miles (8.048 km2). All the islands except Nihoa are north of the Tropic of Cancer, making them the only islands in Hawaii that lie outside the tropics.

The United States Minor Outlying Islands is a statistical designation defined by the International Organization for Standardization's ISO 3166-1 code. The entry code is ISO 3166-2:UM. The minor outlying islands and groups of islands consist of eight United States insular areas in the Pacific Ocean and one in the Caribbean Sea.