Union City is a city in Alameda County, California, United States in the San Francisco Bay Area. It is located approximately 19 miles (31 km) south of Oakland, 30 miles (48 km) from San Francisco, and 20 miles (32 km) north of San Jose. It was incorporated in 1959, combining the communities of Alvarado and Decoto. The city has 72,000 residents across a diverse population. Alvarado is a California Historical Landmark (#503). The city celebrated its 50th anniversary in 2009.

According to the United States Census Bureau, the city has a total area of 19 square miles (49 km2), all land with no bay frontage. The Niles Cone aquifer, managed by the Alameda County Water District, supplies much of the water consumed by Union City. Hayward surrounds the city to the north.

According to the 2020 census estimate, the median income for a household in the city is $142,374. Males have a median household income of $59,274 versus $40,625 for females. The per capita income for the city was $31,411. About 4.5% of the population were below the poverty line, including 5.9% of those under age 18 and 6.2% of those age 65 or over.

Costanoans ("coastal peoples") were the first inhabitants, living along Alameda and Dry Creeks. Shell mounds along the sloughs of Alameda Creek near the Alvarado sugar mill contained burial sites.: 7–9  : 8–9 

The first non-native community was founded in 1850 by John and William Horner, who named it "Union City" after their Sacramento River Steamship, "The Union". In 1854, it merged with the nearby community of New Haven (founded 1851) to form the town of Alvarado, named after a former Mexican governor, Juan Bautista Alvarado. Alvarado was the first county seat of Alameda County, a designation it lost in 1865 to San Leandro. Further east, Decoto was founded in 1870 as a railroad hub, hosting the first transcontinental railroad. Alvarado-Niles Road, one of the city's largest streets, connects the historical Fremont district of Niles with the historical Union City district of Alvarado.

The California Beet Sugar Company, the US' first successful sugar beet mill, was located in Alvarado and began operations in 1870. The plant was demolished in 1977.

Union City is a former railroad and steel town with an extensive industrial heritage. The Pacific States Steel Company occupied the land behind Union City Station that is redeveloping into the Union City Station District Downtown. The Alvarado and Decoto neighborhoods were both former railroad hubs and active railroad lines bisect both East End and the West Side of town. Trains are a way of life in Union City and natives are accustomed to waiting for Amtrak and freight trains to cross while commuting.

In the 1950s, Alvarado and Decoto – the latter making up the eastern side of the town – were annexation targets of the nearby communities of Newark, Hayward, and what would become Fremont. On January 13, 1959, they decided to incorporate into a single city, and named it after the Horners' original settlement, Union City. Tom Kitayama served as the city's first mayor in 1959 and was involved in Union City politics until his 1991 retirement. The population grew from 6,000 in 1959 to 70,000 in 2020. In 2016, Union City started to rebrand and unveiled a new city seal. Union City opened a new Teen Center in 2018.

Union City is the location of the American Licorice Company's West Coast operations, which moved there in 1971.

Union City is home to three major health care providers: a Kaiser Permanente facility, the Tiburcio Vasquez Health Center, and Washington Hospital's Nakamura Clinic.

Union City hosts industrial and shipping companies, including R&S Manufacturing, RCD Concrete, Jatco, and EntirelyPets.

Union Landing Shopping Center is a 100-acre (40 ha) shopping center, adjacent to Interstate 880. It is one of the largest centers in the city and supports about 70 stores. The mall was completed in 1999. The land was previously a drive-in movie park.

A Target shopping center sits near the Hayward/Union City border.

According to Union City's June 2019 Comprehensive Annual Financial Report, the top employers in the city were:

In 2014 New Haven Unified School District enrolls 11,756 students from Union City and Hayward. The district consists of seven (K-5) elementary schools, two (6-8) middle schools, one comprehensive high school (James Logan High School) with approximately 3,400 students enrolled. In addition, James Logan houses a 3,000-seat gymnasium and a 4,000 seat football stadium with lights. Lastly one adult/K-12 independent study school program is available.

In December 2015, New Haven Unified School District renamed Alvarado Middle School to Itliong-Vera Cruz Middle School in honor of Philip Vera Cruz and Larry Itliong. The school district added solar power at school sites to cut energy costs. An arts center and new track and field are available at James Logan High School.

New Haven Unified School District's first teacher strike came in 2019.

Cornerstone International College, located at 725 Whipple Rd., is the city's first post-secondary institution.

Union City lies within the Ohlone Community College District and Chabot-Las Positas Community College District.

Several transit systems serve Union City, including AC Transit, the Dumbarton Express, Bay Area Rapid Transit (BART), and Union City Transit. The latter started in 1974 and runs nine bus lines throughout Union City and parts of Hayward.

BART came to Union City when the system opened in 1972. In 2007 the Union City BART station was rebuilt and developed into Union City Intermodal Transit Station.

Union City is served by a network of high-capacity streets, with four exits on Interstate 880 (Nimitz Freeway). Highway 238 also serves the city (Mission Boulevard). Quarry Lakes Parkway, formerly known as the East–West Connector, is a 3-mile (4.8 km) roadway project that connects Mission Boulevard and Paseo Padre Parkway.

Union City invested $100 million into an expansion of the downtown district, including development surrounding the BART station, which links BART with rail services: Capitol Corridor, Dumbarton Rail Corridor and Altamont Corridor Express (ACE).

Union City runs a council–manager government. The City Council consists of five representatives on a part time basis. On November 3, 2020, the city switched from citywide elections to districts. The Mayor is elected at large. As of December 2023 the elected officials were: Singh, Jaime Patiño, Jeff Wang, and Scott Sakakihara. The mayor and elected council members serve a 4-year term, with a 3-term limit. A utility tax was passed in November 2020.

In 2017, Union City had 35,857 registered voters with 19,644 (54.8%) registered as Democrats, 4,200 (11.7%) registered as Republicans, and 10,693 (29.8%) registered as decline to state voters.

The monthly East Bay Echo and bi-weekly Tri-City Voice newspapers offer local news. Union City Patch served the community and is part of Patch.com.

The first sugar beet factory in the United States was located in Union City, called the California Beet Sugar Company. It is noted by a California Historical Landmark #768, situated at Dyer and Ratekin.: 78 

Bay Area Flight 93 Memorial is in Sugar Mill Landing Park. It was the first monument completed in the United States designed to honor the passengers and crew of United Airlines Flight 93, which was bound for San Francisco, but was hijacked and crashed in rural Pennsylvania on September 11, 2001.

Alameda County's first courthouse was located in Alvarado, starting on June 6, 1853. The original courthouse was a two-story wooden building that was originally a mercantile that included a post office. It was built by A. M. Church and Henry C. Smith. In 1865 the county seat moved to San Leandro. With the widening of Union City Blvd., the original site was paved over.: 78  The site is listed on the California Historical Landmarks list.

Masonic Home at Union City, a senior living community for Masons and their spouses, has as its centerpiece a large brick administration building. The administration building was identified as a significant historic property in the 1974 Historic Resource Inventory of Washington Township. Interior features include a main staircase with stained glass windows, a parlor filled with antiques, and paintings of fourteen Masonic US presidents. The original five-story brick building formed the center of the Administration Building. It was constructed in 1898 and was the Masonic Widows and Orphans Home overlooking Decoto.: 87  The north and south wings of the Administration Building were added in 1914 and 1928, respectively. Ten more buildings were built on this 305-acre (1.2 km2) campus, including Acacia Creek, a continuing care retirement community for all seniors, both Masons and non-Masons, completed in 2010.