At the time Lythronax was announced, news sites reported size estimates of about 7.3–8 m (24–26 ft) in length and around 2.5 t (5,500 lb) in weight, based on comparisons to the much larger relative Tyrannosaurus; Loewen stated that it may have grown even larger.
American paleontologist Gregory S. Paul gave a lower estimate of 5 m (16 ft) in length and a weight of only 500 kg (1,100 lb) in 2016.
Lythronax was a relatively robust tyrannosaurid.
Like other members of the group, it would have possessed small, two-fingered forelimbs, large and strong hindlimbs, broad jaws, and a very robustly constructed skull.
Although earlier small-bodied members of the superfamily Tyrannosauroidea possessed protofeathers, their presence could have varied between species or the age of an individual.

Lythronax had a relatively short snout and a broad skull (width over 40% of the length), as in other tyrannosaurids.
The nasal bones along the top of the snout were much wider at the front than the middle, unlike in other tyrannosaurids.
Viewed from above, the outer margins of the skull (formed by the maxilla and jugal bones) were strongly sigmoid-shaped (or s-shaped).
Along with the width of the frontal bone (a bone at the top of the skull), this appeared to have made the rear part of Lythronax's skull very broad, with orbits (eye sockets) that faced nearly forwards.
These features are otherwise only known in Tarbosaurus and Tyrannosaurus; earlier-diverging tyrannosaurids had less forward-facing orbits, and the rears of their skulls were narrower.

Lythronax was also distinct in that the surfaces of the frontal bone that contacted the prefrontal and postorbital bones at its front and rear sides were separated by only a narrow groove.
The maxillae of Lythronax were robust and strongly convex along their outer margins, as in all other known tyrannosaurids, but differed in their sigmoid-shaped margins.
Lythronax had 11 alveoli (tooth sockets) in each maxilla, a trait shared with no tyrannosaurs other than Teratophoneus and Bistahieversor (other tyrannosaurs had 12 or more maxillary alveoli).
The maxillary teeth were heterodont (differentiated), the first five being much larger than those following.
Some of the frontmost teeth were almost 13 cm (5 in) long.
The teeth were similar to bananas in shape, robust, and serrated.
As in Tyrannosaurus, the shelf of the palate was well developed.

The jugal bone (or "cheek" bone) was robust, and had a broad postorbital process (which projected upwards from the jugal to contact the postorbital bone), unlike other tyrannosaurs except Bistahieversor, Tyrannosaurus and Tarbosaurus.
The front border of the postorbital process had a strong process which indicates that Lythronax had a large subocular flange (a projection into the lower part of the orbit), dissimilar to the smaller ones of other tyrannosaurids.
Each ramus of the dentary (half of the tooth-bearing portion of the lower jaw) was strongly concave towards the outer side (bowing inwards along the length of the skull).
This mirrored the contours of the maxilla of the upper jaw, and the strong expansion of the rear skull; this was similar to Bistahieversor, Tyrannosaurus, and Tarbosaurus, but unlike other tyrannosauroids.
The dentary was also deep at the rear end, indicating that the following part of the mandible was comparable to Tarbosaurus and Tyrannosaurus in depth, but not to other tyrannosaurids.
Like other tyrannosaurids, the surangular bone behind the dentary had a deep and well-developed shelf just in front of where the jaw articulated with the skull, and Lythronax was similar to Tyrannosaurus in that this shelf had a concave upper surface.

Though the postcranial skeleton of Lythronax is poorly known, the known remains of the pubis (part of the pelvis) and the hindlimb show features typical within Tyrannosauridae.
The pubic boot, an expansion on the lower end of the pubis, had a large forward-directed process as in all tyrannosaurids.
In Lythronax, the pubic boot was large and comparatively deep, most similar to those of Tarbosaurus and Tyrannosauru
At the time Lythronax was announced, news sites reported size estimates of about 7.3–8 m (24–26 ft) in length and around 2.5 t (5,500 lb) in weight, based on comparisons to the much larger relative Tyrannosaurus; Loewen stated that it may have grown even larger. American paleontologist Gregory S. Paul gave a lower estimate of 5 m (16 ft) in length and a weight of only 500 kg (1,100 lb) in 2016. Lythronax was a relatively robust tyrannosaurid. Like other members of the group, it would have possessed small, two-fingered forelimbs, large and strong hindlimbs, broad jaws, and a very robustly constructed skull. Although earlier small-bodied members of the superfamily Tyrannosauroidea possessed protofeathers, their presence could have varied between species or the age of an individual.
Lythronax had a relatively short snout and a broad skull (width over 40% of the length), as in other tyrannosaurids. The nasal bones along the top of the snout were much wider at the front than the middle, unlike in other tyrannosaurids. Viewed from above, the outer margins of the skull (formed by the maxilla and jugal bones) were strongly sigmoid-shaped (or s-shaped). Along with the width of the frontal bone (a bone at the top of the skull), this appeared to have made the rear part of Lythronax's skull very broad, with orbits (eye sockets) that faced nearly forwards. These features are otherwise only known in Tarbosaurus and Tyrannosaurus; earlier-diverging tyrannosaurids had less forward-facing orbits, and the rears of their skulls were narrower. 
Lythronax was also distinct in that the surfaces of the frontal bone that contacted the prefrontal and postorbital bones at its front and rear sides were separated by only a narrow groove. The maxillae of Lythronax were robust and strongly convex along their outer margins, as in all other known tyrannosaurids, but differed in their sigmoid-shaped margins. Lythronax had 11 alveoli (tooth sockets) in each maxilla, a trait shared with no tyrannosaurs other than Teratophoneus and Bistahieversor (other tyrannosaurs had 12 or more maxillary alveoli). The maxillary teeth were heterodont (differentiated), the first five being much larger than those following. Some of the frontmost teeth were almost 13 cm (5 in) long. The teeth were similar to bananas in shape, robust, and serrated. As in Tyrannosaurus, the shelf of the palate was well developed.
The jugal bone (or "cheek" bone) was robust, and had a broad postorbital process (which projected upwards from the jugal to contact the postorbital bone), unlike other tyrannosaurs except Bistahieversor, Tyrannosaurus and Tarbosaurus. The front border of the postorbital process had a strong process which indicates that Lythronax had a large subocular flange (a projection into the lower part of the orbit), dissimilar to the smaller ones of other tyrannosaurids. Each ramus of the dentary (half of the tooth-bearing portion of the lower jaw) was strongly concave towards the outer side (bowing inwards along the length of the skull). This mirrored the contours of the maxilla of the upper jaw, and the strong expansion of the rear skull; this was similar to Bistahieversor, Tyrannosaurus, and Tarbosaurus, but unlike other tyrannosauroids. The dentary was also deep at the rear end, indicating that the following part of the mandible was comparable to Tarbosaurus and Tyrannosaurus in depth, but not to other tyrannosaurids. Like other tyrannosaurids, the surangular bone behind the dentary had a deep and well-developed shelf just in front of where the jaw articulated with the skull, and Lythronax was similar to Tyrannosaurus in that this shelf had a concave upper surface.
Though the postcranial skeleton of Lythronax is poorly known, the known remains of the pubis (part of the pelvis) and the hindlimb show features typical within Tyrannosauridae. The pubic boot, an expansion on the lower end of the pubis, had a large forward-directed process as in all tyrannosaurids. In Lythronax, the pubic boot was large and comparatively deep, most similar to those of Tarbosaurus and Tyrannosauru
Lythronax