The Pacific temperate rainforests of western North America is the largest temperate rain forest region on the planet as defined by the World Wildlife Fund (other definitions exist). The Pacific temperate rainforests lie along the western side of the Pacific Coast Ranges along the Pacific Northwest Coast of North America from the Prince William Sound in Alaska through the British Columbia Coast to Northern California, and are part of the Nearctic realm, as also defined by the World Wildlife Fund. The Pacific temperate rain forests are characterized by a high amount of rainfall, in some areas more than 300 cm (10 ft) per year and moderate temperatures in both the summer and winter months (10–24 °C or 50–75 °F).

This ecoregion is a subregion of the Cascadia bioregion.

These rainforests occur in a number of ecoregions, which vary in their species composition, but are predominantly of conifers, sometimes with an understory of broadleaf trees, ferns and shrubs. In the WWF's system, the ecoregions of the Pacific temperate rainforests are the Northern Pacific coastal forests, Haida Gwaii, British Columbia mainland coastal forests, Central Pacific coastal forests, Central and Southern Cascades forests, Klamath-Siskiyou forests, and Northern California coastal forests ecoregions.

The forests in the north contain predominantly Sitka spruce (Picea sitchensis) and western hemlock (Tsuga heterophylla), while those in the coastal forests are home to both species mentioned, as well as coast redwood (Sequoia sempervirens), coast Douglas-fir (Pseudotsuga menziesii), western redcedar (Thuja plicata) and shore pine (Pinus contorta). Notably, many of the world's largest and tallest tree species are found in this ecoregion. Dense growths of epiphytes and mosses cover the trees, and lush vegetation is present everywhere.

Hardwood trees such as the bigleaf maple and the alder are also common, especially at lower elevations and along stream banks, and are vital to the ecosystem, in part because of their nitrogen fixing.

About 200 million years ago (during the Triassic and Jurassic periods), the landscape was dominated by conifers, which were the most diverse group of trees and constituted the greatest majority of large trees. When flowering plants emerged (in the following Cretaceous period), they quickly prevailed, causing most conifers to become extinct, and those that survived to adapt to harsh conditions. Perhaps the most significant difference in this change is that the primitive conifers invested their energy in the basic food supply for every seed, with no certainty of fertilization; by contrast, flowering plants create the food supply for a seed only after it is triggered by fertilization. The Pacific temperate rainforests now remains the only region on Earth of noteworthy size and significance where, due to unique climatic conditions, the conifers flourish as they did before being displaced by flowering plants. The northern Pacific temperate rain forests are relatively young, emerging in the past few thousand years following the retreat of the ice sheets of the last ice age.

The ecosystem of Pacific temperate rainforests is so productive that the biomass on the best sites is at least four times greater than that of any comparable area in the tropics. In sheer mass of living and decaying material - trees, mosses, shrubs, and soil - these forests are more massive than any other ecosystem on the planet. In part, this is due to the rarity of fire. Unlike drier forests, which burn periodically, temperate rain forests are naturally subject to only small-scale disturbances, such as blow-downs and avalanches.

This rain forest spans a wide range of latitude - from about 40 degrees north to about 60 degrees north. The differences in climate from south to north create several major forest zones, characterized by different species.