Rancho Santa Margarita is a city in Orange County, California, United States. One of Orange County's youngest cities, Rancho Santa Margarita is a master-planned community. The population was 47,853 at the 2010 census, up from 47,214 at the 2000 census. Although it is named for Rancho Santa Margarita y Las Flores, which was in San Diego County, the city limits fall within the borders of Rancho Mission Viejo. At 20 characters long (22 including spaces), it is the longest city name in California.

The area is the ancestral lands of the Acjachemen people. The village of Alume was located in Rancho Santa Margarita, between Trabuco Creek and Tijeras Creek. In 1810, the Trabuco Adobe was constructed near the village as an outpost of Mission San Juan Capistrano.

The city seal has the brands of Rancho Mission Viejo and Rancho Santa Margarita and Las Flores on the border, with artwork containing Santiago Peak in the background. The tower in the foreground symbolizes the Rancho Santa Margarita Lake Tower.

Hughes Aircraft Company's Microelectronic Systems Division moved to Rancho Santa Margarita in May 1988 from Irvine. In August 1992, the Hughes plant closed its facilities and moved the division to Carlsbad, California due to budget constraints in the aerospace industry.

Rolling Hills Estates had the longest city name in California with 19 letters until January 1, 2000, when the title was ceded to Rancho Santa Margarita (20 letters) upon the latter's incorporation.

The City is a general law city and operates under the council-manager form of government. Rancho Santa Margarita is a contract city. Police services are provided through contract with the Orange County Sheriff. Fire Protection services are provided through the Orange County Fire Authority.

Rancho Santa Margarita is located at 33°38′29″N 117°35′40″W / 33.64139°N 117.59444°W / 33.64139; -117.59444 (33.641518, -117.594524). It occupies much of a high plateau known as Plano Trabuco.

According to the United States Census Bureau, the city has a total area of 13.0 square miles (34 km2). 13.0 square miles (34 km2) of it is land and 0.04 square miles (0.10 km2) of it (0.27%) is water.

Rancho Santa Margarita is bordered by the city of Mission Viejo on the west, the census-designated places Coto de Caza and Las Flores on the south, the unincorporated Trabuco Canyon on the north, and the Cleveland National Forest on the east.

Vehicular access is provided by California State Route 241 (a toll road), in addition to several surface streets. Santa Margarita Parkway and Antonio Parkway are the city's major thoroughfares, with Avenida De Las Banderas, Avenida Empresa, Avenida De Las Flores, and Alma Aldea being minor thoroughfares. In addition, Alicia Parkway terminates in the city providing access to Mission Viejo.

The majority of the neighborhoods in Rancho Santa Margarita are maintained by larger homeowners associations including SAMLARC, Dove Canyon, Rancho Cielo, Robinson Ranch, Walden and Trabuco Highlands. Dove Canyon, Trabuco Highlands, Robinson Ranch, Walden and Rancho Cielo were all established before Rancho Santa Margarita was an incorporated city. East of Plano Trabuco Road is designated with a Trabuco Canyon (92679) zip code even though the area falls within the City of Rancho Santa Margarita boundary.

Rancho Santa Margarita, like most of southern California, generally has a Mediterranean climate.

According to the City's 2020 Comprehensive Annual Financial Report, the top 10 employers in the city are:

The 2020 United States Census reported a population of 48,119. The racial makeup was 75.5% White, 2.2% African American, 10.9% Asian, and 21.7% Hispanic or Latino of any race.

The 2010 United States Census reported that Rancho Santa Margarita had a population of 47,853. The population density was 3,683.1 inhabitants per square mile (1,422.1/km2). The racial makeup of Rancho Santa Margarita was 37,421 (78.2%) White (67.0% Non-Hispanic White), 887 (1.9%) African American, 182 (0.4%) Native American, 4,350 (9.1%) Asian, 102 (0.2%) Pacific Islander, 2,674 (5.6%) from other races, and 2,237 (4.7%) from two or more races. Hispanic or Latino of any race were 8,902 persons (18.6%).

31.8% of the population possessed a bachelor's degree, with 16.4% possessing a Graduate or Professional Degree. The educational attainment level in Rancho Santa Margarita significantly exceeds the averages throughout the rest of California.

The Census reported that 47,851 people (100% of the population) lived in households, 2 (0%) lived in non-institutionalized group quarters, and 0 (0%) were institutionalized.

There were 16,665 households, out of which 7,699 (46.2%) had children under the age of 18 living in them, 10,144 (60.9%) were opposite-sex married couples living together, 1,703 (10.2%) had a female householder with no husband present, 700 (4.2%) had a male householder with no wife present. There were 747 (4.5%) unmarried opposite-sex partnerships, and 103 (0.6%) same-sex married couples or partnerships. 3,199 households (19.2%) were made up of individuals, and 761 (4.6%) had someone living alone who was 65 years of age or older. The average household size was 2.87. There were 12,547 families (75.3% of all households); the average family size was 3.33.

The population was spread out, with 13,879 people (29.0%) under the age of 18, 3,793 people (7.9%) aged 18 to 24, 13,706 people (28.6%) aged 25 to 44, 13,764 people (28.8%) aged 45 to 64, and 2,711 people (5.7%) who were 65 years of age or older. The median age was 36.0 years. For every 100 females, there were 95.6 males. For every 100 females age 18 and over, there were 92.1 males.

There were 17,260 housing units at an average density of 1,328.4 per square mile (512.9/km2), of which 11,906 (71.4%) were owner-occupied, and 4,759 (28.6%) were occupied by renters. The homeowner vacancy rate was 1.2%; the rental vacancy rate was 5.6%. 35,737 people (74.7% of the population) lived in owner-occupied housing units and 12,114 people (25.3%) lived in rental housing units.

During 2009–2013, Rancho Santa Margarita had a median household income of $104,113, with 3.9% of the population living below the federal poverty line.

In the California State Legislature, Rancho Santa Margarita is in the 38th Senate District, represented by Democrat Catherine Blakespear, and in the 71st Assembly District, represented by Republican Kate Sanchez.

In the United States House of Representatives, Rancho Santa Margarita is in California's 40th congressional district, represented by Republican Young Kim.

According to the California Secretary of State, as of February 10, 2019, Rancho Santa Margarita has 28,462 registered voters. Of those, 11,877 (41.73%) are registered Republicans, 7,511 (26.39%) are registered Democrats, and 7,819 (27.47%) have no political party preference/are independents.

Rancho Santa Margarita has voted Republican in every presidential election since the city's incorporation.

The city is served by Saddleback Valley Unified School District and the Capistrano Unified School District.

The television series The Real Housewives of Orange County, although based in Coto De Caza, is mainly filmed in Rancho Santa Margarita where many of the housewives do business, shopping, commuting, dentistry, and dining.

The Dove Canyon, Rancho Cielo, and Robinson Ranch neighborhoods, while all part of the city, have a zip code of 92679, although the 92688 ZIP code is far more common in the Rancho Santa Margarita area.