Nemesio Rubén Oseguera Cervantes (Latin American Spanish: [neˈmesio oseˈɣeɾa seɾˈβantes]; born 17 July 1966), commonly referred to by his alias El Mencho ([el ˈmentʃo]), is a Mexican drug lord and top leader of the Jalisco New Generation Cartel (CJNG), an organized crime group based in Jalisco. He is the most wanted person in Mexico and one of the most wanted in the U.S. The US government and the Mexican government are offering rewards of US$15 million and MXN$300 million, respectively, for information leading to his arrest.

He is wanted for drug trafficking, organized crime involvement, and undocumented possession of firearms. El Mencho is allegedly responsible for coordinating global drug trafficking operations. Under his command, the CJNG became one of Mexico's leading criminal organizations.

Born into poverty in Mexico, El Mencho grew avocados and dropped out of primary school before immigrating illegally to the U.S. in the 1980s. After being arrested several times, he was deported to Mexico in the early 1990s and worked for the Milenio Cartel. He eventually climbed to the top of the criminal organization and founded the CJNG after several of his bosses were arrested or killed.

His notoriety is also a result of his aggressive leadership and sensationalist acts of violence against both rival criminal groups and Mexican security forces alike. These attacks brought him increased government attention and an extensive manhunt. Security forces suspect he is hiding in the rural terrains of Jalisco, Michoacán, Nayarit, and/or Colima, and is guarded by mercenaries with former military training.

In February 2022 unconfirmed reports began to surface stating that El Mencho had died from respiratory arrest while undergoing treatment in a private hospital in Guadalajara.

However, U.S. Drug Enforcement Administration agent Kyle Mori, who heads the search for El Mencho, denied rumors of his death in an interview he gave to KFI AM's in March 2023.