The blue-throated mountaingem, also known as the blue-throated mountain-gem or blue-throated hummingbird is a species of hummingbird in tribe Lampornithini of subfamily Trochilinae. It is found in the United States and Mexico. The blue-throated mountaingem is the largest hummingbird found in the USA. It is 11.2 to 12.8 cm (4.4 to 5.0 in) long. Males weigh an average of 8.4 g (0.30 oz) and females 6.8 g (0.24 oz). Both sexes have a medium-length black bill, though there is some variation among the subspecies and females' bills are longer than males'. Both sexes of all subspecies have a conspicuous white stripe behind the eye and a narrower stripe extending backward from the corner of its bill under a blackish cheek patch.

The nominate subspecies is the largest and has the longest bill. Adult males have an iridescent cobalt to cerulean blue gorget with a narrow buffy gray margin. They have mostly bright greenish bronze upperparts that become dark bronzy olive on the rump. They have medium brownish gray underparts with some greenish bronze iridescence on the sides of the breast. The tail is black with some faint indigo iridescence and white tips on the outer two pairs of feathers. Females have entirely medium gray underparts without the gorget.

Scientific name: Lampornis clemenciae

Mass: 0.27 oz (Adult)

Conservation status: Least Concern (Population increasing)