It was formally described by palaeontologist Jeffrey A. Wilson and colleagues in 2003 based on a partial skeleton comprising the braincase, spine, hip bone, legs, and tail–a first for an Indian theropod.

The remains of Rajasaurus were found in this fossil-rich limestone bed to which GSI geologist Suresh Srivastava was assigned to excavate on two separate trips from 1982–1983 and 1983–1984.

Fragments of Rajasaurus were also found near Jabalpur in Madhya Pradesh in the northern part of the Lameta Formation, namely a piece of the upper jaw.

Unlike in other theropods but similar to Majungasaurus, the crista prootica, which is typically a ridge along the otic capsule bone in the ear, more so juts outward as a stump.

[2] Wilson, in 2003, assigned Rajasaurus to the subfamily Carnotaurinae, being more closely related to abelisaurids like Majungasaurus and the South American Carnotaurus than to African abelisaurids–as Africa, he believed, had separated from Gondwana first, and South America, India, and Madagascar were connected via Antarctica–on the basis of several similarities such as the presence of a sagittal crest, neck vertebrae with two air pockets, the configuration of the nasal bones, a fleshy growth ("excrescence") on the frontal bone, and a thick skull roof.

[3][12][13] In 2014, the subfamily Majungasaurinae was erected by palaeontologist Thierry Tortosa to separate the newly discovered European Arcovenator, Majungasaurus, Indosaurus, Rahiolisaurus, and Rajasaurus from South American abelisaurids based on physical characteristics such as elongated antorbital fenestrae in front of the eye sockets, and a sagittal crest that widens into a triangular surface towards the front of the head.

Despite large oceanic barriers, it has been suggested that a migration of abelisaurids took place in the Late Cretaceous between Africa, Europe, Madagascar, and India which ultimately isolated South American abelisaurids; it is possible that migration occurred between Europe and India across Africa given its proximity to both, and the volcanic Dras-Kohistan island arc may have allowed island hopping and an indirect path to Asia, though these are still questionable explanations.

[14][15] The following cladogram was recovered by Tortosa (2014):[14] Ceratosaurus Noasauridae Carnotaurus Abelisaurus Rajasaurus Majungasaurus Indosaurus Rahiolisaurus Arcovenator Rajasaurus is distinguished from other genera by its single forehead horn (though Majungasaurus also only has one), the elongated supratemporal fenestrae (holes in the upper rear of the skull), and the ilia bones on the hip which feature a ridge separating the brevis shelf from the hip joint.

[4] Head-butting, inferred from the lack of shock-absorbing cancellous bone in the skull, could have been both or either low-motion shoving matches like modern day marine iguanas (Amblyrhynchus cristatus), or focused on the neck and the flank like giraffes (Giraffa spp.).

[7][16] Rajasaurus has been found in the Lameta Formation, a rock unit radiometrically dated to the Maastrichtian age of the latest Cretaceous representing an arid or semi-arid landscape with a river flowing through it–probably providing shrub cover near the water–which formed between episodes of volcanism in the Deccan Traps.

[23] India, by the Late Cretaceous, had separated from Madagascar and South America during the break-up of Gondwana, and Rajasaurus lived on an isolated island, likely causing endemism and unique characteristics not seen in other abelisaurids.

[1][24] However, despite being an island, there is no evidence of endemic animals with unique traits from Late Cretaceous India, perhaps indicating a continued connection to other parts of the world, likely Africa due to its proximity.

[contradictory][25] The similarity between European and Indian sauropod egg taxa suggests an inter-continental migration of animals between India, Europe, and South America during the Cretaceous, despite water barriers.

[26] Several dinosaurs have been described from the Lameta Formation, such as the noasaurid Laevisuchus; abelisaurids Indosaurus, Indosuchus, Lametasaurus, and Rahiolisaurus; and the titanosaurian sauropods Jainosaurus, Titanosaurus, and Isisaurus.

[30] The Indian theme park Adlabs Imagica features the Rajasaurus River Adventure, a water ride following the fictional "Dr. Roy" through prehistoric times.

[31] Rajasaurus also appeared in the "Badlands" episode of Prehistoric Planet, where at least two individuals are shown hunting down Isisaurus hatchlings as they pass through the volcanic fields of the Deccan Traps.