Patos Island is a small island in the San Juan Islands of the U.S. state of Washington. Since 1893, it has been home to the Patos Island Lighthouse, guiding vessels through Boundary Pass between Canada and the United States. From 1939 to 1978, the island was owned and operated by the United States Coast Guard, with members manning both the lighthouse and the station full-time.

The island and adjacent islets comprise Patos Island Marine State Park, a 207-acre (0.84 km²) marine park with 20,000 feet (6,100 m) of saltwater shoreline. The Washington State Parks and Recreation Commission began operating Patos Island as a state park under a lease agreement with the Bureau of Land Management in 1974. The entire island is owned by the federal government and is administered by the Bureau of Land Management's Wenatchee Office. The Washington State Parks and Recreation Commission operates a small campground facility at Active Cove near the west side of the island, maintains a 1.5-mile (2.4 km) loop trail, and has two offshore mooring buoys.

The bedrock outcrops of Patos Island are made up of rock that formed from hardened sediment which was deposited about 50 million years ago, during the Eocene. The sediments were left behind by a river that carried minerals and rocks from eastern Washington, prior to the formation of the Cascade Mountains. The climate along this river was tropical, with the presence of forests indicated by fossilized wood typical of the Chuckanut Formation. While much of the shale from these geologic processes has eroded, the more-resistant sandstone and conglomerate forms the basis of the island's ridge lines. The island has a varied terrain, with forested rolling hills, rock and sand beaches, and flat, rocky ledges.

Off the southwestern shore of Patos Island, separated by 50 feet (15 m) of water, there is a smaller 15 acres (6.1 ha) island called Little Patos Island. The sheer cliff faces and densely forested surface make this smaller island inaccessible.

Prior to European colonization, Patos Island served as a seasonal fishing, hunting, and camping ground for the Lummi (Xwlemi) and Samish (Xws7ámesh) people. The Samish called the island Tl’x’óy7ten in the Samish dialect of North Straits Salish, meaning "Place of Harvesting Oysters." Throughout the 19th and 20th centuries, the Lummi and Samish continued to use the island to fish despite increased European presence and fishery restrictions.

The name Patos comes from the Spanish pato, meaning "duck," which was given to the island in 1792 by Commander Dionisio Alcalá Galiano of the Sutil and Captain Cayetano Valdés y Flores of the Mexicana. Galiano and Valdés later shared their charts with Captain George Vancouver, who was surveying the area on behalf of the United Kingdom. Charles Wilkes of the 1841 United States Exploring Expedition listed it as Gourd Island, but the United States Coast Survey and the British Admiralty each retained the Spanish name on their charts in 1854 and 1874, respectively. Patos Island was among the contested islands in the 1859 Pig War between the United States and the United Kingdom.

While conducting hydrographic surveys for the U.S. Coast Survey from 1857 to 1860, Captain James Alden lent his name to Alden Point, at the island's western edge; Alden also named nearby Active Cove, between Patos and Little Patos Islands, after his vessel, the USCS Active. The British Admiralty christened the eastern cape Toe Point in 1858.

The island was established as a lighthouse reservation by the U.S. federal government in 1875, prior to the ultimate construction of a light station in 1893. Upon the creation of the lighthouse, the island became an attraction for the public, with several hundred visiting each year, often staying with the lighthouse keepers or camping on the island. The second lighthouse keeper, Edward Durgan, brought his wife and children to the island, including his daughter Helen, who would go on to write The Light on the Island about her time growing up on Patos Island.