Washington is the northwesternmost state of the contiguous United States. It borders Idaho to the east, bounded mostly by the meridian running north from the confluence of the Snake River and Clearwater River (about 117°02'23" west), except for the southernmost section where the border follows the Snake River. Oregon is to the south, with the Columbia River forming the western part and the 46th parallel forming the eastern part of the Oregon–Washington border. During Washington's partition from Oregon, the original plan for the border followed the Columbia River east until the confluence with the Snake river, and then would have followed the Snake River east; this was changed to keep Walla Walla's fertile farmland in Washington.

To the west of Washington lies the Pacific Ocean. Its northern border lies mostly along the 49th parallel, and then via marine boundaries through the Strait of Georgia, Haro Strait, and Strait of Juan de Fuca, with the Canadian province of British Columbia to the north.

Washington is part of a region known as the Pacific Northwest, a term which always refers to at least Washington and Oregon, and may or may not include some or all the following, depending on the user's intent: Idaho, western Montana, northern California, British Columbia, and Alaska.

The high mountains of the Cascade Range run north–south, bisecting the state. In addition to Western Washington and Eastern Washington, residents call the two parts of the state the "Westside" and the "Eastside", "Wet side" and "Dry side", or "Timberland" and "Wheatland", the latter pair more commonly in the names of region-specific businesses and institutions. These terms reflect the geography, climate, and industry of the land on both sides of the Cascades.

From the Cascade Mountains westward, Western Washington has a mostly Mediterranean climate, with mild temperatures and wet winters, autumns and springs, and relatively dry summers. The Cascade Range has several volcanoes, which reach altitudes significantly higher than the rest of the mountains. From north to south, these major volcanoes are Mount Baker, Glacier Peak, Mount Rainier, Mount St. Helens, and Mount Adams. All are active volcanoes.

Mount Rainier—the tallest mountain in the state— is 50 miles (80 km) south of the city of Seattle, from which it is prominently visible. The U.S. Geological Survey considers 14,411-foot-tall (4,392 m) Mount Rainier the most dangerous volcano in the Cascade Range, due to its proximity to the Seattle metropolitan area, and most dangerous in the continental U.S. according to the Decade Volcanoes list. It is also covered with more glacial ice than any other peak in the contiguous 48 states.

Western Washington also is home of the Olympic Mountains, far west on the Olympic Peninsula, which support dense forests of conifers and areas of temperate rainforest. These deep forests, such as the Hoh Rainforest, are among the only rainforests in the continental United States. While Western Washington does not always experience a high amount of rainfall as measured in total inches of rain per year, it does consistently have more rainy days per year than most other places in the country.

Eastern Washington—the part of the state east of the Cascades—has a relatively dry climate, in distinct contrast to the west side. It includes large areas of semiarid steppe and a few truly arid deserts in the rain shadow of the Cascades; the Hanford reservation receives an average annual precipitation of 6 to 7 inches (150 to 180 mm). Despite the limited amount of rainfall, agriculture is an extremely important business throughout much of Eastern Washington, as the soil is highly productive and irrigation, aided by dams along the Columbia River, is fairly widespread. The spread of population in Eastern Washington is dominated by access to water, especially rivers. The main cities are all located alongside rivers or lakes; most of them are named after the river or lake they adjoin.