The Mariana gray swiftlet has sooty black upper parts with a slightly paler rump. The underparts are dark gray but with a brownish tinge. Some white is present at the base of the feathers in the loreal region. The tarsi are naked and the irises are dark hazel. The plumage of both sexes is alike. Weights of 21 birds averaged 7.4 grams (range = 6.4 to 9.0 grams). Other measurements include: wing, average of 107.6 mm and 108 mm; tail, average of 52.3 mm; exposed culmen, average of 4.0 mm; tarsus, average of 10.4 mm, and wingspan, maximum of 233 mm. Wing and tail length are >94% of adult size at fledging.