A city in and the county seat of Sweetwater County, Wyoming, United States, in the southwestern part of the state. The population was 12,515 at the 2010 census.

Green River was incorporated in 1868 in what was then the Dakota Territory. The city was the starting point from which John Wesley Powell started his famous expeditions of the Green River, the Colorado River, and the Grand Canyon in the late 1800s. Green River was originally supposed to be the site of a division point for the Union Pacific Railroad, but when the railroad finally reached the point, officials were surprised to find that a city had already been established there. They moved the division point 12 miles (19 km) west to Bryan, Wyoming.

At the time of its incorporation in 1868, Green River had about 2000 residents and permanent adobe buildings were being built. However, when the division point of the railroad was moved west, the city shrank to a mere 101 residents. Just when the city was on the verge of shrivelling up, the Black's Fork dried up during a drought and the railroad was forced to move the division point back to Green River. The town was officially incorporated under the new laws of Wyoming on May 5, 1891.

The city is known as being one of the first in the United States to ban door-to-door solicitation; see Green River Ordinance.

The Green River Basin contains the world's largest known deposit of trona ore. Soda ash mining from trona veins 900 and 1,600 feet (490 m) deep is a major industrial activity in the area, employing over 2000 persons at five mines. The mining operation is less expensive for production of soda ash in the United States than the synthetic Solvay process, which predominates in the rest of the world. The trona in Sweetwater County was created by an ancient body of water known as Lake Gosiute. Over time, the lake shrank. With the loss of outflows, highly alkaline water (salt brine) began to evaporate, depositing the beds of trona.

The Green River Basin also has large oil shale and natural gas reserves, which remain virtually untouched due to the high cost of extracting the oil from the hard shale formations. However, an increase in oil prices in 2008 and a national desire to become more energy independent led to an increase in well drilling and oil exploration. Expansion growth from Halliburton and Exxon, as well as other oil companies, created a mini-boom for Green River and its sister city, Rock Springs.

As of the census of 2000, there were 11,808 people, 4,177 households, and 3,212 families residing in the city. The population density was 861.5 people per square mile (332.5/km²). There were 4,426 housing units at an average density of 322.9 per square mile (124.6/km²). The racial makeup of the city was 92.13% White, 0.27% African American, 1.36% Native American, 0.32% Asian, 0.06% Pacific Islander, 4.23% from other races, and 1.63% from two or more races. Hispanic or Latino of any race were 10.21% of the population.

There were 4,177 households out of which 42.5% had children under the age of 18 living with them, 64.4% were married couples living together, 8.4% had a female householder with no husband present, and 23.1% were non-families. 19.3% of all households were made up of individuals and 6.0% had someone living alone who was 65 years of age or older. The average household size was 2.80 and the average family size was 3.22.

In the city the population was spread out with 31.1% under the age of 18, 8.8% from 18 to 24, 28.9% from 25 to 44, 24.8% from 45 to 64, and 6.5% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 102.8 males. For every 100 females age 18 and over, there were 100.7 males.

The median income for a household in the city was $53,164, and the median income for a family was $59,100. Males had a median income of $51,418 versus $24,306 for females. The per capita income for the city was $20,398. About 3.1% of families and 4.5% of the population were below the poverty line, including 6.0% of those under age 18 and 5.3% of those age 65 or over.