The Kansas Jayhawks, commonly referred to as simply KU or Kansas, are the athletic teams that represent the University of Kansas. KU is one of three schools in the state of Kansas that participate in NCAA Division I. The Jayhawks are also a member of the Big 12 Conference. KU athletic teams have won fifteen national championships all-time, with twelve of those being NCAA Division I championships: four in men's basketball, one in men's cross country, three in men's indoor track and field, three in men's outdoor track and field, and one in women's outdoor track and field. Kansas basketball also won two Helms Foundation National Titles in 1922 and 1923, and KU Bowling won the USBC National Title in 2004.

The name "Jayhawk" comes from the Kansas Jayhawker militias during the Bleeding Kansas era of the American Civil War. The term became part of the lexicon of the Missouri-Kansas border in about 1858, during the Kansas territorial period, as these militia groups began to grow in Kansas.

Another historian of the territorial period described the Jayhawkers as bands of men that were willing to fight, kill, and rob for a variety of motives, including defense against pro-slavery "Border Ruffians", abolition of slavery, driving pro-slavery settlers from Kansas Territory and their claims of land, Christianity, revenge, or plunder and personal profit.

In September 1861, the town of Osceola, Missouri, was burned to the ground by Jayhawkers during the Sacking of Osceola. On the 150th anniversary of that event in 2011, the town asked the University of Kansas to remove the Jayhawk as its mascot; although, the university refused.

Over time, proud of their state's contributions to the end of slavery and the preservation of the Union, Kansans embraced the "Jayhawker" term. The term came to be applied to people or items related to Kansas. When the University of Kansas fielded their first football team in 1890, like many universities at that time, they had no official mascot. They used many different independent mascots, including a pig. During the 1890s, the team was referred to as the Jayhawkers by the student body. Over time, the name was gradually supplanted by its shorter variant, and KU's sports teams are now almost exclusively known as the Jayhawks. The Jayhawk appears in several Kansas cheers, most notably, the "Rock Chalk, Jayhawk" chant in unison before and during games. In the traditions promoted by KU, the jayhawk is said to be a combination of two birds, "the blue jay, a noisy, quarrelsome thing known to rob other nests; and the sparrow hawk, a stealthy hunter."

The term Jayhawker was made famous in Clint Eastwood’s movie The Outlaw Josey Wales. An older Kansas couple comes into a general store in Texas. The checker says, “The wheat is from Kansas and the molasses comes from Missouri.” Grandma says, “Well sir we'll do without the molasses, anything from Missouri has a taint about it.” Grandpa says, “Now, Grandma, you’ve got to tread lightly now that we’re here in Texas. Lot of nice elements from Missouri coming West.” Grandma responds, “Never heard of nice things from Missouri coming West, and treading lightly is not my way. We’re from Kansas, Jayhawkers and proud of it.”

The link between the term "Jayhawkers" and any specific kind of mythical bird, if it ever existed, had been lost or at least obscured by the time KU's bird mascot was invented in 1912. The originator of the first bird mascot, Henry Maloy, struggled for over two years to create a pictorial symbol for the team, until hitting upon the bird idea. As explained by Mr. Maloy, "the term ‘jayhawk’ in the school yell was a verb and the term ‘jayhawkers’ was the noun." KU's current Jayhawk tradition largely springs from Frank W. Blackmar, a KU professor. In his 1926 address on the origin of the Jayhawk, Blackmar specifically referenced the blue jay and sparrow hawk. Blackmar's address served to soften the link between KU's athletic team moniker and the Jayhawkers of the Kansas territorial period, and helped explain the relatively recently invented Jayhawk pictorial symbol with a myth that appears to have been of even more recent fabrication. More recently, however, the University and KU fans have again embraced the history of the Jayhawker moniker, with the football team, among other Varsity teams, donning civil war themed uniforms.

Jayhawks Big Jay and Baby Jay are the costume mascots used by the University of Kansas.