The blue-throated mountaingem, also known as the blue-throated mountain-gem or blue-throated hummingbird (Lampornis clemenciae) is a species of hummingbird in tribe Lampornithini of subfamily Trochilinae. It is found in the United States and Mexico.

In the 19th century the blue-throated mountaingem was placed several different genera, and in the early 20th century in its own genus Cyanolaemus. Since the mid-1900s it has been in its present genus Lampornis. It has three subspecies, the nominate L. c. clemenciae, L. c. phasmorus, and L. c. bessophilus.

The blue-throated mountaingem is the largest hummingbird found in the United States. 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.

Subspecies L. c. phasmorus is the smallest and has the shortest bill. Its upperparts are bright green rather than greenish bronze. Males' underparts are a cold gray rather than brownish gray and females' are dark gray. The iridescence on the sides of the breast is green. The white tips on the tail feathers are wider than those of the nominate.

Subspecies L. c. bessophilus is between the other two subspecies in size. Its upperparts are duller than the nominate's, with less bronze to the green. Males' underparts are brownish gray and females' medium pale gray; like the nominate there is some greenish bronze iridescence on the sides of the breast. The white tips on the tail feathers are the narrowest of all subspecies.

The nominated subspecies of blue-throated mountain gem has the largest range. It is found in Mexico's Sierra Madre Oriental and central plateau as far south as Oaxaca. L. c. basophils is found in southeastern Arizona and in the northwestern Mexican states of Sonora, Chihuahua, and Durango. L. c. phasmorus is positively known only from the Chisos Mountains of southern Texas, where it breeds, but its non-breeding range is not known. Birds in the northern Mexican state of Nuevo León may also be this subspecies rather than the nominate.

The blue-throated mountain gem is found in a variety of moist forest landscapes. In the US and northern Mexico, it occurs in riparian forest (often in canyons), pine-oak forest, and mixed coniferous forests. In central and southern Mexico it tends to favor coniferous forests. In Arizona it is found in the "sky island" mountain ranges, seldom below 1,300 m (4,300 ft) of elevation. Near Mexico City it occurs between 3,600 and 3,900 m (11,800 and 12,800 ft) and in Oaxaca between 2,500 and 3,000 m (8,200 and 9,800 ft).