The Allen's Hummingbird, Selasphorus sasin, is a species of hummingbird, a family of small passerine birds.

The Allen's Hummingbird is a small bird, with mature adults reaching only 3 to 3½ inches in length. The male Allen's has a green back and forehead, with rust-colored rufous flanks, rump, and tail. The male's throat is also an iridescent orange-red. The female and immature Allen's Hummingbirds are similarly colored, but lack the iridescent throat patch, instead having a series of speckles on their throat. Females are mostly green, featuring rufous colors only on the tail, which also has white tips. The female and immature Allen's Hummingbirds are so similar to the female Rufous Hummingbird that the two are almost indistinguishable in the field. Both species' breeding seasons and ranges are common factors used to differentiate between the two species in a particular geographical area.