Alectrosaurus (/əˌlɛktroʊˈsɔːrəs/; meaning "alone lizard") is a genus of tyrannosauroid theropod dinosaur that lived in Asia during the Late Cretaceous period, about some 96 million years ago in what is now the Iren Dabasu Formation.

It was a medium-sized, moderately-built, ground-dwelling, bipedal carnivore, estimated at 5 to 6 m (16 to 20 ft) with a body shape similar to its much larger advanced relative, Tyrannosaurus.

Alectrosaurus was a very fast running tyrannosauroid as indicated by the elongated hindlimbs that likely filled the niche of a pursuit predator, a trait that seems to be lost by the advanced and robust tyrannosaurids, in adulthood.

In 1923, the Third Asiatic Expedition of the American Museum of Natural History, led by chief paleontologist Walter W. Granger, was hunting for dinosaur fossils in Mongolia.

On April 25 in the gobi desert, assistant paleontologist George Olsen excavated and recovered the holotype AMNH FARB 6554, a nearly complete right hindlimb.

This specimen included a right humerus, two incomplete manual digits, four fragmentary caudal vertebrae, and other two or three unspecified elements that were discarded due to bad preservation.

Observing similarities with the hindlimbs of specimen AMNH 5664 Gorgosaurus sternbergi, he classified this new genus as a "Deinodont", a term that is now considered equivalent to tyrannosaurid.

[12] Alectrosaurus was originally characterized as a long-armed theropod, but Perle 1977 and Mader & Bradley 1989 observed that the forelimbs of the specimen AMNH 6368 did not belong to the genus, as they do not share characteristics with Tyrannosauroidea, and assigned them to the Therizinosauria incertae sedis.

These traits are found in terrestrial runner birds, suggesting that Alectrosaurus was suited as a fast-running tyrannosauroid dinosaur with well developed hindlimbs; probably a pursuit predator.

By comparing the limbs of numerous theropods they noted that most tyrannosauroids were highly cursorial and leggy animals, with the exception of giant and stocky-legged forms such as Tarbosaurus or Tyrannosaurus.

[10] In 2001, a study conducted by Bruce Rothschild and colleagues, examined 23 foot bones referred to Alectrosaurus for signs of stress fractures, but none were found.

[6] Contemporaneous paleofauna from this formation included other theropods such as Archaeornithomimus, Caenagnathasia, Erliansaurus, Gigantoraptor and Neimongosaurus; the sauropod Sonidosaurus and the two hadrosauroids Bactrosaurus and Gilmoreosaurus.

[21][22][23] Alexander Averianov and Hans-Dieter Sues in 2012 estimated that the Upper Bayan Shireh correlated the Iren Dabasu Formation and both had similar environments.

In 2015, a giant caenagnathid was reported from the Bayan Shireh Formation, preserving a partial lower rostrum (beak) that is extremely similar to that of Gigantoraptor.