Haplocheirus is a genus of theropod dinosaur. Initially, it was thought to be the oldest alvarezsauroid, predating all other members by approximately 63 million years. However, subsequent research has cast doubt on this classification. In 2019, a study suggested that Haplocheirus might belong to the compsognathid group rather than being an alvarezsauroid. Additionally, a 2022 study proposed that it shares more similarities with ornithomimosaurs.

Haplocheirus possessed an enlarged thumb claw, akin to alvarezsaurids, but it retained two other functional fingers, which was a departure from alvarezsaurids where only the thumb was notably large and clawed. With its long legs, it was likely a swift runner and had relatively large pupils for an animal of its size. According to one study, both Haplocheirus and the alvarezsaurid Shuvuuia were probably creatures of the night.

As an alvarezsaur, it represented one of the larger members of this group, with an estimated adult body mass of about 41 kilograms, although it was outweighed by Bonapartenykus. The holotype, which was a juvenile still in the growth stage, would have measured approximately 190–230 centimeters in total length, depending on the tail's length.