Julián Castro (/ˌhuːliˈɑːn/ HOO-lee-AHN, Spanish: [xuˈljan]; born September 16, 1974) is an American lawyer and politician from San Antonio, Texas. A member of the Democratic Party, he was the youngest member of President Obama's cabinet, serving as the 16th United States secretary of housing and urban development from 2014 to 2017. Castro served as the mayor of his native San Antonio, Texas from 2009 until he joined Barack Obama's cabinet in 2014.

Castro was mentioned as a possible running mate for Hillary Clinton during the 2016 presidential campaign. He is the twin brother of Congressman Joaquin Castro. On January 12, 2019, Castro launched his campaign for the Democratic nomination for President of the United States in 2020 in San Antonio. He dropped out of the presidential race on January 2, 2020, endorsing the candidacy of Elizabeth Warren soon after.

Castro was born in San Antonio, Texas, the son of Maria "Rosie" Castro and Jessie Guzman. He is the identical twin brother of current United States Representative Joaquin Castro; Julián was born a minute before Joaquin; they were born at 2:40 and 2:41 am, respectively.

Castro is of Mexican descent. His mother is a Chicana political activist who helped establish the Chicano political party La Raza Unida, and who ran for the San Antonio City Council in 1971. Castro once stated, "My mother is probably the biggest reason that my brother and I are in public service. Growing up, she would take us to a lot of rallies and organizational meetings and other things that are very boring for an 8-, 9-, 10-year-old". His father, Jessie Guzman, is a retired mathematics teacher and political activist. Never married, Rosie and Jessie separated when Castro and his brother were eight years old. Castro's Texan roots trace back to 1920, when his grandmother Victoria Castro joined extended family members there as a six-year-old orphan from northern Mexico.

Castro attended Thomas Jefferson High School in San Antonio, where he played football, basketball and tennis; he also collected trading cards.^[which?] He skipped his sophomore year and graduated in 1992, ranking ninth in his class. He had received an offer to play tennis at Trinity University, an NCAA Division III school in his hometown, but chose to attend Stanford University, along with his twin brother Joaquin.

Castro graduated from Stanford in 1996 with a bachelor's degree in political science and communications. He said he began thinking about entering politics while at Stanford, where he and his brother launched their first campaigns and won student senate seats, tying for the highest number of votes. Castro has credited affirmative action for his admission into Stanford, telling The New York Times, "Joaquin and I got into Stanford because of affirmative action. I scored 1210 on my SATs, which was lower than the median matriculating student. But I did fine in college and in law school. So did Joaquin. I'm a strong supporter of affirmative action because I've seen it work in my own life". Between his sophomore and junior years, Castro worked as an intern at the White House during the presidency of Bill Clinton.

Castro entered Harvard Law School in 1997 and graduated with a Juris Doctor in 2000. His brother graduated from both schools with him. After law school, the two brothers worked for the law firm Akin Gump Strauss Hauer & Feld before starting their own firm in 2005.

In 2018, Castro was named as the Dean's Distinguished Fellow and Fellow of the Dávila Chair in International Trade Policy at the Lyndon B. Johnson School of Public Affairs.