Kenomagnathus (meaning "gap jaw", in reference to the diastema in its upper tooth row) is a genus of synapsid belonging to the Sphenacodontia, which lived during the Pennsylvanian subperiod of the Carboniferous in what is now Garnett, Kansas, United States. It contains one species, Kenomagnathus scottae, based on a specimen consisting of the maxilla and lacrimal bones of the skull, which was catalogued as ROM 43608 and originally classified as belonging to "Haptodus" garnettensis. Frederik Spindler named it as a new genus in 2020.

Norman Newell discovered a fossil locality near Garnett, Kansas, United States in 1931, belonging to the Rock Lake Member of the Stanton Formation. Around 1932, Henry Lane and Claude Hibbard had collected a variety of animal and plant fossils from the locality. Among these were skeletons of Petrolacosaurus, which were subsequently described in 1952 by Frank Peabody. Hoping to find more material, a field team from the University of Kansas Natural History Museum conducted further excavations in 1953 and 1954; they found trackways, coelacanth fish, several additional Petrolacosaurus skeletons, and "pelycosaur" (early-diverging synapsid) fossils representing three genera. One of these was a partial skeleton that distinctly differed from the others; when Peabody reported on these discoveries in 1957 paper, he observed that the skeleton was of a primitive sphenacodontid, but deferred its description to a later time.

In 1965, Robert Carroll found another articulated partial skeleton in the same locality. Philip Currie attributed this skeleton to the primitive sphenacodontid of Peabody in 1977, and recognized it as a new species of Haptodus, which he named Haptodus garnettensis. However, up until that point, all specimens of H. garnettensis were either badly crushed or immature. Throughout the 1980s, a number of additional specimens were discovered at the locality, including adult and subadult specimens. This allowed Michel Laurin to identify distinguishing characteristics for H. garnettensis and to incorporate it into a phylogenetic analysis, which found it to be outside the Sphenacodontidae. He published these results in 1993. Among the additional specimens was a partial skull consisting of a left maxilla and lacrimal, which were catalogued in the Royal Ontario Museum (ROM) as ROM 43608.

Analyses of specimens assigned to H. garnettensis by Frederik Spindler and colleagues later suggested that there was not one but between four and six distinct taxa among them, which primarily differed in their jaws and teeth. They also recognized differences between "H." garnettensis and the type species of Haptodus, H. baylei. In a 2013 conference presentation, Spindler, Kirstin Brink, and Graciela Piñeiro suggested that this variation was based on diet, making these taxa a prehistoric analogue of Darwin's finches. Spindler formally named ROM 43608 as belonging to a new genus and species in 2020, which he named Kenomagnathus scottae. The generic name Kenomagnathus is derived from the Greek words κένωμα ("gap") and γνάθος ("jaw"), referencing the diastema (gap) in its tooth row. Meanwhile, the specific name scottae honours Diane Scott, a fossil preparator at the University of Toronto Mississauga who "greatly helped with teaching and specimen handling", inspired Spindler's research, and further prepared the specimen in 2013.

Among close relatives, Kenomagnathus can be distinguished by its tall snout, judging by the maxilla and especially the lacrimal. The projection at the front of the lacrimal would have formed a large part of the rear border of the bony nostrils. In "H." garnettensis, the lacrimal still contributed to the border, but with a narrow projection. From below, the upward projection at the front of the maxilla would also have contributed to the border of the nostrils, but the angle of this projection differed from "H." garnettensis. The height of the lacrimal, which bordered the front of the eye socket, also implies that Kenomagnathus had large eyes.

The tooth-bearing bottom margin of the maxilla in Kenomagnathus was more convex than "H." garnettensis, and is unique in that it lacked a concave region (or "precanine step"). Another distinguishing characteristic is the diastema, a toothless region spanning the width of three teeth at the front of the maxilla, where the bone noticeably thinned and could not have borne tooth sockets. Behind the diastema were two precanine teeth, two large canine teeth, and at least fourteen post-canine teeth (eleven being preserved). There were two weakly thickened regions, or buttresses, on the maxilla, with one above the canines and one further back. Due to the shortness of the maxilla, the canines were located further forward than in close relatives. Like "H." garnettensis, Kenomagnathus had tall and nearly straight teeth, with striations on the inner surfaces of the teeth reaching the tips, but those of Kenomagnathus were more slender and blunter at the tip.

In 2020, Spindler identified three characteristics that placed Kenomagnathus in the Sphenacodontia: the blunt teeth, the convex bottom margin of the maxilla, and the height of the lacrimal and the upward projection of the maxilla. However, based on the contribution of the lacrimal to the border of the bony nostrils, Kenomagnathus was excluded from the more restrictive group Sphenacodontoidea. Within this evolutionary grade of "haptodontine" sphenacodontians, the fragmentary nature of specimens has complicated the resolution of their relationships. This is exacerbated by the fact that many "haptodontines" are very similar to each other save for differences in their teeth and skull proportions.

For his 2015 thesis, Spindler conducted a preliminary phylogenetic analysis of "haptodontines", based on a new set of characteristics that he assembled to resolve their relationships. After defining the specimens associated with "H." garnettensis as opposed to Kenomagnathus and a taxon he named "Tenuacaptor reiszi", he could not resolve whether Kenomagnathus or Ianthodon was more basal (less specialized). Removing "Tenuacaptor" produced a more derived (more specialized) position for both Kenomagnathus and "H." garnettensis inside the Sphenacodontia. Phylogenetic trees illustrating both phylogenetic hypotheses are shown below.