Westminster is a city in western Orange County, California, United States. Westminster was founded in 1870 by Rev. Lemuel Webber as a Presbyterian temperance colony and was incorporated in 1957.

Westminster is bordered by the city of Seal Beach on the west, by Garden Grove on the north and east, and by Huntington Beach and Fountain Valley on the south. Santa Ana, the county seat of Orange County, is east of Westminster.

Westminster has one of the largest Vietnamese populations in the US. They settled largely in Little Saigon, and the city is known as the "capital" of overseas Vietnamese with 39,799 Vietnamese Americans, or 43.8% of the total population, as of the 2020 United States Census.

In the court case Mendez v. Westminster (1947), a Hispanic man sued the Westminster School District for forcing his daughter, Sylvia Mendez, to attend a school for Mexican children. They eventually won and thus began the process of desegregation. Sylvia Mendez was later awarded the Presidential Medal of Freedom by President Barack Obama on February 15, 2011.

Westminster won the All-America City Award in 1996.

Westminster was founded in 1870 by Rev. Lemuel Webber as a Presbyterian temperance colony. The name is derived from the Westminster Assembly of 1643, which established the basic tenets of the Presbyterian faith. During the early years of its history, farmers refused to harvest grapes since they associated grapes with alcohol.

Westminster was incorporated in 1957, at which time it had 10,755 residents. Originally, the city was named Tri-City because it was to be the amalgamation of three cities: Westminster, Barber City, and Midway City. Midway City ultimately refused incorporation, leaving only Barber City to be absorbed into the newly incorporated Westminster. The former Barber City was located in the western portion of the current city of Westminster.

According to the United States Census Bureau, the city has a total area of 10.0 square miles (26 km2), all land. The San Diego Freeway and a short segment of the Garden Grove Freeway pass through Westminster.

Westminster is bordered by the city of Seal Beach on the west, by Garden Grove on the north and east, and by Huntington Beach and Fountain Valley on the south. Santa Ana, the county seat of Orange County, is east of Westminster, although a small portion of Garden Grove lies between the two cities. Westminster borders the unincorporated area of Midway City, except for a small portion where Midway City borders Huntington Beach on the south. The Little Saigon district of the town is mostly situated on Bolsa Avenue between Magnolia Street and Brookhurst Street.

At the 2010 census Westminster had a population of 89,701. The population density was 8,926.5 inhabitants per square mile (3,446.5/km2). The racial makeup of Westminster was 32,037 (35.7%) White (25.6% Non-Hispanic White), 849 (0.9%) African American, 397 (0.4%) Native American, 42,597 (47.5%) Asian, 361 (0.4%) Pacific Islander, 10,229 (11.4%) from other races, and 3,231 (3.6%) from two or more races. Hispanic or Latino of any race were 21,176 persons (23.6%). A total of 36,058 residents were of Vietnamese ancestry (40.2% of city residents), the highest concentration of Vietnamese Americans of any community in the United States.

The census reported that 89,031 people (99.3% of the population) lived in households, 381 (0.4%) lived in non-institutionalized group quarters, and 289 (0.3%) were institutionalized.

There were 26,164 households, 10,759 (41.1%) had children under the age of 18 living in them, 14,986 (57.3%) were opposite-sex married couples living together, 3,681 (14.1%) had a female householder with no husband present, 1,810 (6.9%) had a male householder with no wife present. There were 1,114 (4.3%) unmarried opposite-sex partnerships, and 151 (0.6%) same-sex married couples or partnerships. 4,247 households (16.2%) were one person and 2,170 (8.3%) had someone living alone who was 65 or older. The average household size was 3.40. There were 20,477 families (78.3% of households); the average family size was 3.74.

The age distribution was 20,920 people (23.3%) under the age of 18, 8,568 people (9.6%) aged 18 to 24, 24,065 people (26.8%) aged 25 to 44, 23,356 people (26.0%) aged 45 to 64, and 12,792 people (14.3%) who were 65 or older. The median age was 38.7 years. For every 100 females, there were 97.8 males. For every 100 females age 18 and over, there were 96.4 males.

There were 27,650 housing units at an average density of 2,751.5 per square mile, of the occupied units 15,135 (57.8%) were owner-occupied and 11,029 (42.2%) were rented. The homeowner vacancy rate was 1.5%; the rental vacancy rate was 7.3%. 51,408 people (57.3% of the population) lived in owner-occupied housing units and 37,623 people (41.9%) lived in rental housing units.

During 2009–2013, Westminster had a median household income of $52,633, with 16.7% of the population living below the federal poverty line.

At the 2000 census there were 88,207 people in 26,406 households, including 20,411 families, in the city. The population density was 8,724.2 inhabitants per square mile (3,368.4/km2). There were 26,940 housing units at an average density of 2,664.5 units per square mile (1,028.8 units/km2). The racial makeup of the city was 45.79% White, 0.99% African American, 0.61% Native American, 38.13% Asian, 0.46% Pacific Islander, 10.19% from other races, and 3.84% from two or more races. Hispanic or Latino of any race were 21.70%.

Of the 26,406 households 37.8% had children under the age of 18 living with them, 58.4% were married couples living together, 12.4% had a female householder with no husband present, and 22.7% were non-families. 16.9% of households were one person and 7.4% were one person aged 65 or older. The average household size was 3.32 and the average family size was 3.71.

The age distribution was 25.9% under the age of 18, 8.8% from 18 to 24, 32.6% from 25 to 44, 21.5% from 45 to 64, and 11.2% 65 or older. The median age was 34 years. For every 100 females, there were 99.9 males. For every 100 females age 18 and over, there were 97.9 males.

The median household income was $49,450 and the median family income was $54,399. Males had a median income of $37,157 versus $28,392 for females. The per capita income for the city was $18,218. About 10.7% of families and 13.5% of the population were below the poverty line, including 18.0% of those under age 18 and 7.9% of those age 65 or over.

In the California State Legislature, Westminster is in the 36th Senate District, seat currently vacant, and in the 70th Assembly District, represented by Republican Tri Ta.

In the United States House of Representatives, Westminster is in California's 45th congressional district, represented by Republican Michelle Steel.

It has a city council form of local government.

According to the city's 2017 Comprehensive Annual Financial Report, the top employers in the city are:

The city's major shopping mall is Westminster Mall, which consists of more than 180 stores. The mall is located south of the 405 freeway, between Goldenwest Street and Edwards Street. Westminster's Little Saigon community is home to the Asian Garden Mall (Phước Lộc Thọ), a large Asian mall.

Since joining the Sequential Brands company, the DVS Shoes footwear brand relocated from Torrance, California to Westminster. The company's headquarters is located on Fenwick Lane.

Four school districts have boundaries that cover parts of Westminster:

Huntington Beach Union HSD operates Westminster High School, while Garden Grove USD operates La Quinta High School.

Some sections of HBUHSD Westminster are zoned to Ocean View High School.