Gastineau Channel is a channel between the mainland of the U.S. state of Alaska and Douglas Island in the Alexander Archipelago of southeastern Alaska. It separates Juneau on the mainland side from Douglas (now part of Juneau), on Douglas Island. The first European to sight the channel was Joseph Whidbey early in August 1794, first from the south and later from the west. It was probably named for John Gastineau, an English Civil Engineer and Surveyor.