The Azure-rumped Tanager is a medium-sized tanager. Genders are not distinguishable from a distance: both have a primarily blue plumage with distinct dark markings (lores, lower auriculars, spots on breast), and turquoise edgings on the remiges and rectrices.
The congeners Gray-and-gold (Tangara palmeri) and Beryl-spankled Tanager (T. nigroviridis) are similar in appearance (Hellmayr 1910, Hilty and Simon 1977). Both species, however, are allopatric and occur in South America (Isler and Isler 1999). Within its range, Azure-rumped Tanager is unmistakable under optimal observation conditions. Under unfavorable light conditions and at a distance, it might be confused with Yellow-winged Tanager (Thraupis abbas), especially young birds; Yellow-winged Tanager has similar dark markings (lores and mask, dark scalloping on the back) and utters similar sibilant vocalizations. Also Blue-gray Tanager (Thraupis episcopus) is sympatric, utters similar sibilant vocalizations, and in flight shows a similar silhouette.
Adults: Sexes similar. Crown purplish blue; neck and back greenish with a dark center of each feather causing a mottled appearance; rump sky blue; face pale blue with dark lores and dark lower auriculars; remiges dark edged turquoise; greater coverts black with narrow turquoise edge; median coverts black; rectrices dark edged turquoise; throat, chest and belly pale blue; across the chest a band of dark spots. The extent of the dark mark on the lower auriculars and the dark spotting on the breast is individually variable (Eisermann et al. 2011c).
Juveniles: Resemble adults due to dark markings (lower auriculars, lores, and spots on breast) but overall duller (Eisermann et al. 2011c). The throat is whitish with two fine dark lines, breast and belly whitish (photos in Eisermann et al. 2011c).
Young birds are duller than adults, but they attain the adult-like plumage within a few weeks after fledging (Eisermann et al. 2011c). There is no detailed information available on molt.
Bill: upper mandible dark gray, lower mandible pale gray with dark tip.
Tarsi and toes: pale gray.
Measurements are available only from the type specimen: length: 144.8 mm, wing 86.4 mm, tail 58.4 mm (Sclater 1868, Ridgway 1902).