The Big Thicket is the name given to a somewhat imprecise region of a heavily forested area of Southeast Texas in the United States. This area represents a portion of the mixed pine-hardwood forests or "Piney Woods" of the Southeast US. The National Park Service established the Big Thicket National Preserve (BTNP) within the region in 1974 and it is recognized as a biosphere reserve by UNESCO. Although the diversity of animals in the area is high for a temperate zone with over 500 vertebrates, it is the complex mosaic of ecosystems and plant diversity that is particularly remarkable. Biologists have identified at least eight, and up to eleven, ecosystems in the Big Thicket area. More than 160 species of trees and shrubs, 800 herbs and vines, and 340 types of grasses are known to occur in the Big Thicket, and estimates as high as over 1000 flowering plant species and 200 trees and shrubs have been made, plus ferns, carnivorous plants, and more. The Big Thicket has historically been the most dense forest region in Texas.

Existing literature states that Native Americans were known to have lived and hunted in the area nomadically, but did not establish permanent settlements there before the Alabama–Coushatta settled in the northeast about 1780. However, there is insufficient archaeological evidence to support this claim. What records that do exist could suggest human occupation dating back to the Clovis culture 13,400–12,700 years ago, with numerous era diagnostic points being found in all but one of the counties commonly considered to be in the Big Thicket. Spanish explorers and missionaries had a sporadic presence in the region, however colonization and settlement was not their aim, preferring to establish forts outside of the Region where the French were encroaching from the east (namely around Natchitoches, Nacogdoches, and the lower Trinity river valley). Logging in the late 19th and 20th centuries dramatically reduced the forest concentration. Efforts to save the Big Thicket from the devastation of oil and lumber industries started as early as the 1920s with the founding of the East Texas Big Thicket Association by Richard Elmer Jackson.

Conservatively the area occupies all of Hardin County, most of Polk, and Tyler Counties, and parts of Jasper, Liberty and San Jacinto Counties, including areas between the Neches River on the east, the Trinity River on the west, Pine Island Bayou on the south, to the higher elevations and older Eocene geological formations to the north. Broader interpretations have included the area between the Sabine River on the east and the San Jacinto River on the west including much of Montgomery, Newton, Trinity, and Walker Counties, as well. Several attempts to define the boundaries of the Big Thicket have been made, including a biological survey in 1936 which included over 3,350,000 acres (13,600 km²) covering 14 counties. A later botanical based study in 1972 included a region of over 2,000,000 acres (8,100 km²). This same habitat extends into Louisiana and eastward.

One's fondness for the area is hard to explain. It has no commanding peak or awesome gorge, no topographical feature of distinction. Its appeal is more subtle.

— Big Thicket Legacy, University of Texas Press, 1977

Geology: The Big Thicket is notable for the unusually high diversity of plants concentrated in a relatively small area. A major factor enabling this diversity is an unusually high diversity of soils. The exposed surface soils are relatively recent, late Cenozoic Era, predominantly Miocene, Pliocene, Pleistocene, and Holocene Epoch formations. Other than some slight and uniform tilting, the strata are not greatly deformed. The strata were laid down in several cycles of glaciations and warmer interglacial interludes. Although glaciers never extended as far south as Texas, it was during these cold periods that enough water was frozen in polar regions to lower sea levels and the Big Thicket area was dry land well above sea level. Conversely, during the warmer interludes, enough ice melted to increase sea levels and submerge Southeast Texas in the sea. It was during those warmer periods of submersion that alluvial and delta deposits from the streams and rivers of the continent laid down the succession of strata, each new layer burying and compacting the previous. At some point there was an approximately 1% tilting of the layers. Some say it was caused by the rising of the Rocky Mountains, while others say it was the weight of the successive deposits that eventually caused the Gulf of Mexico to subside. The older Pliocene, Miocene, and Oligocene formations are exposed in the north (Polk, San Jacinto, and Tyler counties), while the more recent Quaternary Period formations are exposed in the south (Hardin and Liberty counties).

Soils: The soils of the various formations at the surface have shifted and intermingled through erosion and other factors over time and can be locally quite complex. The Big Thicket contains a greater variety of soils than any area of comparable size in the United States. Sources vary, but from 50 to 100 soil types are said to occur in Hardin County alone. The surface soils tend to be slightly to strongly acidic, and only occasionally neutral in much of the Big Thicket Basin. However, calcareous or alkaline soils do occur in the region, particularly along the Trinity River, in the Beaumont Formation to the south, the Fleming Formation to the north, and northern areas of the Willis Formation where the layer is thin and erosion and road cuts have exposed the Fleming Formation. Most plants are sensitive to soil pH levels (acidic, neutral, or alkaline) and many species will only grow in areas with a specific soil type. Claude A. McLeod's study stated that the Big Thicket was "an edaphicmesophytic climax forest" – edaphic, meaning the soil is a greater influence on the plants than the climate, and mesophytic meaning a medium moisture level.

Topography: The Big Thicket Basin has been described as shallow bowl or dish, tilted to one side, with the high rim on the interior north and low rim on the coastal south. Eastern and western rims consist of the low ridges that divide the Neches River and Sabine River drainage on the east and the Neches River and Trinity River drainage on the west. Elevations range from about 350 feet (110 m) in the north to 25–35 feet (8–11 m) in the south, dropping as low as 3 feet (1 m) at confluence of the Neches River and Pine Island Bayou. Northern regions are characterized by low gently rolling hills and well-formed drainage systems of streams and creeks in the landscape. The southern regions are comparatively flat, with poor drainage and support moderately extensive wetlands.