Lythronax argestes belongs to the family Tyrannosauridae, a family of large-bodied coelurosaurs; most tyrannosaurid genera are known from North America and Asia.
Based on its stratigraphic position, Lythronax is the oldest tyrannosaurid discovered so far.
Prior to Lythronax being formally named, Zanno and colleagues noted in 2013 that the holotype specimen was likely distinct from Teratophoneus and Bistahieversor, both likewise from southern Utah.
This would mean there were at least three tyrannosaurid genera present in the Western Interior Basin during the Campanian stage.
A phylogenetic analysis conducted by Zanno and colleagues placed all three taxa within a single group of Tyrannosauridae to the exclusion of all other members of the group.

A detailed phylogenetic analysis, conducted by Loewen and colleagues to accompany their 2013 description of Lythronax, based on 303 cranial and 198 postcranial features, placed it and Teratophoneus within the subfamily Tyrannosaurinae.
Lythronax was a sister taxon of a group consisting of the Maastrichtian taxa Tarbosaurus and Tyrannosaurus and the late Campanian Zhuchengtyrannus.
It was more closely related to this group than other taxa such as Daspletosaurus and Teratophoneus, which were younger than Lythronax but older than the group.

In 2017, American paleontologists Stephen Brusatte and Thomas D. Carr published a new phylogenetic analysis of Tyrannosauroidea, including a more comprehensive suite of anatomical characteristics and taxa, that disagreed with the results of Loewen and colleagues.
While the tribe Alioramini was outside Tyrannosauridae in the analysis by Loewen and colleagues, Brusatte and Carr placed that group as the most basal (early-diverging or "primitive") group within Tyrannosaurinae.
Conversely, Loewen and colleagues found Bistahieversor to be a derived ("advanced") tyrannosaurine closely related to the likewise derived Teratophoneus and Lythronax, while Brusatte and Carr placed it in a more basal position directly outside Tyrannosauridae, with both Teratophoneus and Lythronax as basal tyrannosaurines.
It was suggested that both of these results stemmed from an over-weighting of some features by Loewen and colleagues, which resulted in the long-snouted alioramin forms being excluded from the short-snouted tyrannosaurines, and the placement of Bistahieversor and Lythronax closer to Tyrannosaurus than otherwise.
The results of the two contrasting analyses are shown in the cladograms below:


 
Topology 1: Loewen and colleagues, 2013
 
Topology 2: Brusatte and Carr, 2017
 
In a popular book published in 2016, Paul suggested that Lythronax argestes may be a member of the genus Tyrannosaurus, and remarked that derived tyrannosaurids "are being badly oversplit at the genus level".
Subsequent publications—including both taxonomic and phylogenetic analyses—have retained the species in the separate genus Lythronax.
Lythronax argestes belongs to the family Tyrannosauridae, a family of large-bodied coelurosaurs; most tyrannosaurid genera are known from North America and Asia. Based on its stratigraphic position, Lythronax is the oldest tyrannosaurid discovered so far. Prior to Lythronax being formally named, Zanno and colleagues noted in 2013 that the holotype specimen was likely distinct from Teratophoneus and Bistahieversor, both likewise from southern Utah. This would mean there were at least three tyrannosaurid genera present in the Western Interior Basin during the Campanian stage. A phylogenetic analysis conducted by Zanno and colleagues placed all three taxa within a single group of Tyrannosauridae to the exclusion of all other members of the group.
A detailed phylogenetic analysis, conducted by Loewen and colleagues to accompany their 2013 description of Lythronax, based on 303 cranial and 198 postcranial features, placed it and Teratophoneus within the subfamily Tyrannosaurinae. Lythronax was a sister taxon of a group consisting of the Maastrichtian taxa Tarbosaurus and Tyrannosaurus and the late Campanian Zhuchengtyrannus. It was more closely related to this group than other taxa such as Daspletosaurus and Teratophoneus, which were younger than Lythronax but older than the group.
In 2017, American paleontologists Stephen Brusatte and Thomas D. Carr published a new phylogenetic analysis of Tyrannosauroidea, including a more comprehensive suite of anatomical characteristics and taxa, that disagreed with the results of Loewen and colleagues. While the tribe Alioramini was outside Tyrannosauridae in the analysis by Loewen and colleagues, Brusatte and Carr placed that group as the most basal (early-diverging or "primitive") group within Tyrannosaurinae. Conversely, Loewen and colleagues found Bistahieversor to be a derived ("advanced") tyrannosaurine closely related to the likewise derived Teratophoneus and Lythronax, while Brusatte and Carr placed it in a more basal position directly outside Tyrannosauridae, with both Teratophoneus and Lythronax as basal tyrannosaurines. It was suggested that both of these results stemmed from an over-weighting of some features by Loewen and colleagues, which resulted in the long-snouted alioramin forms being excluded from the short-snouted tyrannosaurines, and the placement of Bistahieversor and Lythronax closer to Tyrannosaurus than otherwise. The results of the two contrasting analyses are shown in the cladograms below:


 
Topology 1: Loewen and colleagues, 2013
 
Topology 2: Brusatte and Carr, 2017
 
In a popular book published in 2016, Paul suggested that Lythronax argestes may be a member of the genus Tyrannosaurus, and remarked that derived tyrannosaurids "are being badly oversplit at the genus level". Subsequent publications—including both taxonomic and phylogenetic analyses—have retained the species in the separate genus Lythronax.
Lythronax