KMJ (580 kHz) is a commercial AM radio station in Fresno, California. It airs a news/talk radio format, and simulcasts with sister station KMJ-FM. Owned by Cumulus Media, the studios and offices are located at the Radio City building on Shaw Avenue in North Fresno.

The transmitter site is in Orange Cove, California, on East American Avenue at Cove Road. While AM 580 is a regional broadcast frequency, the station is powered at 50,000 watts, the highest power for an AM station permitted by the Federal Communications Commission (FCC). To protect other stations on AM 580, KMJ uses a four-tower array directional antenna. The signal covers most of Central California and reaches into the Bay Area, Sacramento and Bakersfield. KMJ is Central California's primary entry point station in the Emergency Alert System.

KMJ-AM-FM focus primarily on locally produced talk programming and news on weekdays. Mornings begin with an agricultural news hour, followed by "Fresno's Morning News", a three-hour block of news, sports, traffic and weather. Middays and afternoons feature local talk hosts. Several nationally syndicated programs are carried at night, including Mark Levin, Armstrong & Getty, Red Eye Radio and America in The Morning from Westwood One, a subsidiary of Cumulus Media, the parent company of KMJ-AM-FM.

Weekends feature shows on money, health, real estate, auto repair and dining. Some shows are paid brokered programming. Weekend hosts include Chris Plante, Chad Benson and Ric Edelman. Local newscasters are heard at the beginning of most hours, with Fox News Radio carried at night.

KMJ is one of the oldest radio stations in the United States. On December 1, 1921, the U.S. Department of Commerce, in charge of radio at the time, adopted a regulation formally establishing a broadcasting station category, which set aside the wavelength of 360 meters (833 kHz) for entertainment broadcasts, and 485 meters (619 kHz) for market and weather reports. On March 23, 1922, the San Joaquin Light and Power Corporation was issued a license with the randomly assigned call letters KMJ, for a new Fresno station operating on the 360 meter "entertainment" wavelength. By coincidence, KMJ had previously been the call sign assigned to a US merchant marine ship, the SS Matoa, in World War I. A few months later the station was also authorized to broadcast weather reports on 485 meters. The lack of available broadcasting frequencies meant that many localities had to allocate time slots between multiple timesharing stations. However, because there were no other area stations, KMJ had unrestricted hours. (Until 1936, and the arrival of KARM, KMJ was the only radio station in Fresno.) KMJ remains one of only a few dozen stations to keep its original three-letter call sign from its founding.

In late 1923, KMJ was reassigned to 1100 kHz, which was changed to 1210 in early 1924, to 1280 kHz a year later, and to 820 kHz in mid-1927. On November 11, 1928, as part of the implementation of a major nationwide reallocation under the provisions of the Federal Radio Commission's General Order 40, KMJ was reassigned to a "local" frequency, 1200 kHz. The next year this was changed to another "local" frequency, 1210 kHz.

In 1925 ownership was changed to The Fresno Bee, which was part of the McClatchy Newspaper Company chain of newspapers.

McClatchy was intent on improving KMJ's coverage, and competed with KTAB in Oakland (now KZAC in San Francisco) for reassignment to 580 kHz, a "regional" frequency, which was being made available by the Federal Radio Commission. Eventually, KMJ was awarded the new channel, and the station moved to 580 kHz, effective July 22, 1932, with a power increase from 100 to 500 watts, with unlimited time. Later, power was boosted to 1,000 watts, using a non-directional antenna from a building rooftop in Downtown Fresno.