The Mongols Motorcycle Club, also known as the Mongol Brotherhood or Mongol Nation, is an international outlaw motorcycle club. Originally formed in Montebello, California, in 1969, the club is headquartered in Southern California. Although the Mongols' main presence lies in California, they also have chapters nationwide in 19 states and internationally in 28 countries. Law enforcement officials estimate approximately 5,000 "full-patched" members are in the club. The Mongols are the 3rd -largest outlaw biker club in the world, after the Hells Angels and the Outlaws.

The Mongols are designated an organized crime group by the United States Department of Justice, the Australian Federal Police, and Europol.

The Mongols Motorcycle Club was formed in Montebello, California on December 5, 1969. The club had ten founding members, the majority of whom were Vietnam veterans. The first national president of the Mongols, Louis Costello, named the club in honor of Genghis Khan and the Mongol Empire. The Mongols' original sergeant-at-arms, Alfonso "Big Al" Aceves, served three tours of duty in Vietnam with the 101st Airborne Division. The club's founders were reportedly a group of Hispanic bikers from East Los Angeles who formed the Mongols as an alternative to the Hells Angels, which did not allow non-White members at the time. The Mongols voted to become an "outlaw" club in 1974. The Mongols' membership increased as the club began recruiting inmates, particularly from the California Men's Colony in San Luis Obispo. Within ten years of its foundation, the club had established chapters in Los Angeles, the San Gabriel Valley, the San Fernando Valley, Long Beach, San Diego and Bakersfield.

The Mongols became involved in a long-term feud with the Hells Angels beginning in 1977. The Hells Angels ceded control of much of Southern California to the Mongols during the 1980s on the condition that the Angels would in turn hold rights over Northern California.

In 2000, the Mongols had a membership of around 200, with 21 chapters across the United States. By 2019, the club had expanded across the U.S. and into a dozen countries internationally.

The Mongols' insignia depicts a caricature of a Mongol warrior – reported to be Genghis Khan – sporting a topknot, bell-bottoms and sunglasses, and riding a motorcycle. This image is trademarked and registered with the United States Patent and Trademark Office. The Mongols' logo, along with additional patches, is worn on members' "colors". Other patches worn include those which indicate a member's rank within the club, the location of the chapter to which he belongs, club slogans, and "1%" emblems. Mongols patches utilize a black-and-white color scheme. The Mongols' back patches, or "rockers", are awarded to the club's members in three steps; firstly the bottom "rocker" which indicates the location of the chapter, followed by the club logo, and finally the top "rocker", reading the name of the club and indicating full membership. According to law enforcement, a skull and crossbones patch worn by a Mongol indicates that the member has killed on behalf of the club. Additionally, a tattoo reading "RFFN" (an acronym of "respect few, fear none") is allegedly only permitted to be worn by a Mongol who has committed an act of violence against a member of the Hells Angels. Mongols members are also known to wear Las Vegas Raiders-branded clothing in order to circumvent bans on gang colors.

The club's mottos include: "Live Mongol, die Mongol" ("LMDM"); "Mongols forever, forever Mongols ("MFFM"); and "Respect few, fear none" ("RFFN"). Other names for the club include the Mongol Nation and Mongol Brotherhood. The Mongols' fight song goes as follows:

We are Mongol raiders, we're raiders of the night
We're dirty sons of bitches, we'd rather fuck and fight
Hidy, hidy, Christ Almighty, who the fuck are we?
Shit, fuck, cunt, suck, Mongols MC!