The green-headed hillstar (Oreotrochilus stolzmanni) is a species of hummingbird found in the Andes of southern Ecuador and northern and central Peru. It is one of 6 species in the genus Oreotrochilus, and can be distinguished from its relatives by subtle differences in plumage coloration. The green-headed hillstar received its name due to its bronze and green crown and gorget of bright emerald green plumage. The species was first discovered by English ornithologist Osbert Salvin in 1895 and named after Polish ornithologist Jean Stanislaus Stolzmann.

Hillstars of this genus are distinct among the hummingbird family Trochilidae because of their preference for high-elevation environments, and the development of many behavioral and physiological adaptations in order to survive the cold climate of the high Andes.

The green-headed hillstar is a species of South American hummingbird ranging in length from 12 to 13 cm and weighing between 7.9 and 8.4 g.

The green-headed hillstar is a sexually dimorphic species. The males are more brightly colored than females. Males have a bronzed-green posterior, including the forehead, crown, back, sides and rump. Their wings are a dark contrast to their back. The primaries and secondaries are typically a dark black/brown and green. Their tail's central rectrices are a soft bronze and their wide, blunt ended outer rectrices are nearly white. On their anterior, they have a metallic emerald green gorget that may appear bronze or blue in certain angles. Their underparts are white with a black streak down the center. At certain angles, the gorget of the adult male will have a slight blue gleam. The females share a similar physique to males, primarily differing in gorget and tail feathers. Their gorgets are white with dotted lines of green to grey spots. Females also have darker tails than males. Their tails are green and black with white webbing along the base of the outer rectrices. The wings of adult males are larger than those of adult females by approximately 3.5mm. This is a common trait for species in the hillstar genus, excluding the wedge-tailed hillstar. Juveniles resemble adult females.

There is no significant difference in measurements to the Andean hillstar, Ecuadorian hillstar, black-breasted hillstar or blue-throated hillstar. It is believed this is due to the commonality of a high altitude, cold Andean climate with similar diets, which has stabilized measurements specifically of the size of the bill, wing and rectrices.