CBK (540 kHz) is a Canadian public radio station licensed to Watrous, Saskatchewan. It broadcasts the CBC Radio One network as a Class A clear-channel AM station powered at 50,000 watts around the clock from a non-directional antenna near Watrous.

Its studios are located at the CBC's broadcast centre at 2440 Broad Street in Regina, with an additional bureau in the Saskatoon Co-op building on 4th Avenue South in Saskatoon. The Regina facility also houses CBK-FM and CBKT-DT. In Regina, a nested rebroadcaster, CBKR-FM 102.5 MHz, simulcasts CBK for listeners who may have trouble receiving the 540 AM signal amid downtown office and apartment buildings.

Due to CBK's low frequency, transmitter power, and Saskatchewan's flat land (with excellent ground conductivity), its daytime signal reaches most of the southern two-thirds of Saskatchewan, including Regina, Saskatoon, Yorkton, Swift Current, Lloydminster, Moose Jaw and Prince Albert. It also provides grade B coverage as far west as Calgary and as far east as Winnipeg, and reaches across the border into North Dakota and Montana. At night, it can be heard across much of the western half of North America with a good radio, but is strongest in Western Canada and the North-Central and Northwestern United States.

CBK officially opened during an evening ceremony in Watrous on July 29, 1939. The K in the station's call sign honours Henry Kelsey, an English fur trader, explorer, and sailor who was the first recorded European to have visited what is now Saskatchewan.

CBC engineers deliberately chose to place the station's transmission facilities near Watrous in order to provide the best possible broadcast signal to the densely populated portion of Saskatchewan, including the cities of Regina and Saskatoon. Watrous is located about 150 kilometres (93 mi) northwest of Regina and about 100 kilometres (62 mi) southeast of Saskatoon in an area where potash-rich soil provides especially good ground conductivity, an important component in determining the strength and reach of an AM radio station's daytime ground wave signal. Additionally, Watrous, in particular, was an advantageous location due to being on the main line of the Canadian National Railway, whose telecommunications infrastructure was used to deliver content to CBC radio stations before the creation of the Trans Canada Microwave system.

Because of the factors making the CBK signal particularly strong, the station was originally intended as the CBC's clear-channel station for the Prairies, broadcasting the full CBC radio schedule together with privately owned affiliates in the region broadcasting portions of the schedule (clear-channel CKY in Winnipeg—now CBW—was one such affiliate at the time). The strong daytime signal also spills into the United States and was initially the only radio station receivable during the daytime in parts of North Dakota and Montana.

For most of World War II, CBK aired programming in both English and French. The French programming was prepared at CBK by a two-person crew and included newscasts, musical programs, and transcriptions of CBC programming produced in Montreal.

At the start, CBK had no physical presence in Saskatchewan beyond Watrous. Although privately owned CBC affiliates in the province occasionally originated programming to be aired over the full CBC network including CBK, CBK was initially a pass-through for programming fed from Toronto. Starting in 1948, its programming was fed from Winnipeg. The station's first full production studio within Saskatchewan opened on October 1, 1954, in Regina. An additional studio facility opened in Saskatoon following the sign-on of television station CBKST. In 1980, CBK's regional weekday morning program began to be hosted simultaneously from both Regina and Saskatoon.