No, Ankylosaurus was not a theropod. Ankylosaurus belonged to a group of reptiles known as ankylosaurs, which are part of the greater clade Ornithischia, the bird-hipped dinosaurs. This dinosaur lived during the Late Cretaceous period and is well-known for its heavily armored body and distinctive club-like tail.
Theropods, on the other hand, are a separate group of dinosaurs primarily characterized by their bipedal stance and often carnivorous diet, which includes famous species like Tyrannosaurus rex and Velociraptor. Theropods are more closely related to modern birds than they are to ankylosaurs. Therefore, while both groups are part of the larger dinosaur family, they belong to different classifications within that family.