Siletzia is a massive formation of early to middle Eocene epoch marine basalts and interbedded sediments in the forearc of the Cascadia subduction zone, on the west coast of North America. It forms the basement rock under western Oregon and Washington and the southern tip of Vancouver Island. It is now fragmented into the Siletz and Crescent terranes.

Siletzia corresponds geographically to the Coast Range Volcanic Province (or Coast Range basalts), but is distinguished from slightly younger basalts that erupted after Siletzia accreted to the continent and differ in chemical composition. The Siletzia basalts are tholeiitic, a characteristic of mantle-derived magma erupted from a spreading ridge between plates of oceanic crust. The younger basalts are alkalic or calc-alkaline, characteristic of magmas derived from a subduction zone. This change of composition reflects a change from marine to continental volcanism that becomes evident around 48 to 42 Ma (millions of years ago), and is attributed to the accretion of Siletzia against the North American continent.

Various theories have been proposed to account for the volume and diversity of Siletzian magmatism, as well as the approximately 75° of rotation, but the evidence is insufficient to determine Siletzia's origin; the question remains open.

The accretion of Siletzia against the North American continent approximately 50 million years ago (contemporaneous with the initiation of the bend in the Hawaiian-Emperor seamount chain) was a major tectonic event associated with a reorganization of Earth's tectonic plates. This is believed to have a caused a shift in the subduction zone, termination of the Laramide orogeny that was uplifting the Rocky Mountains, and major changes in tectonic and volcanic activity across much of western North America.

The rock of Siletzia has been exposed in various places by tectonic uplift (as around the periphery of the Olympic Mountains), anticlinal folding (such as the Black Hills and Willapa Hills in southwestern Washington), and overthrusting onto other formations (along various faults in central and southern Oregon). These exposures have been variously named the Metchosin Formation of Vancouver Island, the Crescent Formation, Black Hills Formation (Washington), and Willapa Hills volcanics of Washington, and the Siletz River Volcanics and Roseburg Formation of Oregon. (See map. The Grays River Volcanics of Washington and Tillamook Volcanics of Oregon are now considered post-Siletz.) Elsewhere Siletzia is covered by younger volcanic and sedimentary deposits.

The discovery of Siletzia began in 1906 with Arnold's description and naming of a small exposure on the north side of the Olympic Peninsula near Port Crescent, Washington. Though this exposure is small, he recognized as very likely that much more of it was buried under younger deposits. With recognition that similar rock exposed at other outcrops is part of the same formation, the name Crescent Formation is now generally applied to all early and middle Eocene basalts on the Olympic Peninsula and Puget lowlands.

The Metchosin Igneous Complex at the southern tip of Vancouver Island was described in a series of reports (1910, 1912, 1913, 1917) by Clapp, who recognized it as correlative with the Crescent formation on the other side of the Strait of Juan de Fuca. Weaver recognized that these "Metchosin volcanics" included various Eocene basalts in western Washington and the Oregon Coast Range as far south as the Klamath Mountains. The Siletz River Volcanics was described in 1948 by Snavely and Baldwin after exposures near the Siletz River, Oregon, and the Roseburg and related formations in southern Oregon described in various reports from the 1960s on.

"Siletzia" was coined in 1979 by Irving, (based in turn on the Siletz River and the Siletz Reservation), to describe the full extent of these Eocene basalts and interbedded sedimentary formations.