James Kenneth Hahn (born July 3, 1950) is a Los Angeles Superior Court judge and an American former lawyer and politician. A Democrat, Hahn was elected the 40th mayor of Los Angeles in 2001. He served until 2005, at which time he was defeated in his bid for re-election. Prior to his term as Mayor, Hahn served in several other capacities for the city of Los Angeles, including deputy city attorney (1975–1979), city controller (1981–1985) and city attorney (1985–2001). Hahn is the only individual in the city's history to have been elected to all three citywide offices. He is currently a sitting judge on the Los Angeles County Superior Court.

As Mayor, Hahn appointed Bill Bratton, the former NYPD Commissioner, as Chief of the Los Angeles Police Department, opting not to renew Bernard Parks' second term as chief. Bratton's appointment is widely seen as leading to the sharp declines in Los Angeles' crime rate and improved morale in the department. Hahn also led the successful campaign to defeat secession in the San Fernando Valley, Hollywood, and San Pedro, thereby keeping Los Angeles intact. While he is noted primarily for these two accomplishments, they also helped lead to his unsuccessful re-election bid; African Americans upset at Parks' ousting and San Fernando Valley residents disappointed with the secession verdict had been the two constituencies that had propelled him to victory four years earlier in 2001.

A member of the Hahn family of California, he is the brother of Los Angeles County Supervisor and former Congresswoman Janice Hahn, the nephew of former California State Assemblyman and Los Angeles City Councilman Gordon Hahn and the son of former Los Angeles City Councilman and long-time Los Angeles County Supervisor Kenneth Hahn.

Hahn was born on July 3, 1950, in Los Angeles, the son of Ramona (Fox) and Kenneth Hahn, and was raised in the Morningside Park district of Inglewood near South Los Angeles. Hahn attended Manchester Avenue Elementary School, Daniel Freeman Elementary School, Horace Mann Junior High School, and Los Angeles Lutheran High School.

He graduated from the Los Angeles campus of Pepperdine University in California magna cum laude with a bachelor's degree in English and a minor in journalism in 1972. He received his Juris Doctor (J.D.) degree from the Pepperdine University School of Law in 1975. In 1994, he was selected as the School of Law's distinguished alumnus. While in college, he assisted in the development of a paralegal program for the Family Law Center of the Legal Aid Society and during law school, he clerked for the Los Angeles district attorney's office.

Upon graduation in 1975 until 1979, Hahn worked as a prosecutor and deputy city attorney in the Office of the City Attorney. From 1979 to 1981, he was in private practice with Robert Horner.

In 1981, Hahn was elected the fifth City Controller of Los Angeles and served until 1985. Elected at the age of 30 and sworn in two days before his 31st birthday, Hahn is the youngest person ever elected to that position.

Hahn served from 1985 to 2001 as Los Angeles City Attorney, overseeing one of the largest public law offices in the nation with approximately 450 attorneys. As City Attorney, he worked to rid LA's neighborhoods of gang activity through the use of gang injunctions. He was involved in crafting state legislation regarding gang enforcement known as the Street Terrorism Enforcement and Prevention Act. During Hahn's tenure, he led the litigation to stop the Joe Camel ad campaign and reached a settlement of 312 million dollars for the city. He used those funds to create the Tobacco Enforcement Project to prevent the sale of tobacco to minors. Hahn re-established the domestic violence unit and sponsored over 30 pieces of relevant state legislation, helping to shape California's domestic violence laws.