When the first therizinosaur fossil claws were discovered in 1948, they were found in close proximity to several fragments of large, flattened ribs. Maleev noted a similarity between those ribs and the ribs of certain sea turtles, and envisioned the claws and ribs belonging to a large turtle or turtle-like reptile.

In 1970, A. K. Rozhdestvensky suggested that the claws belonged to a large theropod dinosaur, and later finds of a complete hand, arm and shoulder bore out T. cheloniformis' dinosaurian affinities. Rozhdestvensky speculated that Therizinosaurus was a giant dinosaurian anteater, using its claws to rip open anthills and termite mounds.

The late '70s and early '80s brought the discovery of four different but incomplete dinosaurs called "segnosaurs" (slow lizards). These segnosaurs were typified by wide stomachs and hips, massive, four-toed feet and a cheeked, beaked skull adapted to a vegetarian diet. In 1984, Gregory Paul argued that segnosaurs bridged the gap between primitive sauropodomorphs (dinosaurs that gave rise to the great "Brontosaurus" types) and ornithischians (armored, horned and duckbilled dinosaurs). Noting a similarity between the shoulder blades of Segnosaurus and Therizinosaurus, Paul published a composite segnosaur skeletal reconstruction with giant therizinosaur claws in 1988.

A more conservative view was espoused by Barsbold and Maryanska in 1990, placing the Segnosauria (not including Therizinosaurus) as an unresolved "third party" between the well-established Theropods and Sauropodomorphs. In 1993, Russell and Dong published a description of Alxasaurus, a fairly complete, primitive therizinosauroid that firmly established a relationship between Therizinosaurus and the segnosaurs.

Based largely on skeletal features of Alxasaurus, most dinosaurologists believe the therizinosauroids are highly derived, herbivorous members of the great group of predominantly carnivorous theropods, closely related to either ornithomimosaurs or oviraptorosaurs. This would place the Therizinosauroidea as much more closely related to birds than to any other plant-eating dinosaurs. The recent discovery of a Chinese therizinosaur preserved with proto-feathers has borne out this theory.