Like all species of Dacnis the Scarlet-breasted Dacnis is small, active, with a thin pointed bill, but the plumage is perhaps the most distinctive in the genus. Both sexes are adorned with scarlet cummerbunds fading orange to yellow below, and both have conspicuous yellow irises. The adult male is primarily hyacinth blue dorsally as well as on the upper breast and throat. On the mantle and back these blue feather are silvery-blue along the shaft giving them a streaked appearance.
The adult male is unmistakable, but reminiscent of the Blue-backed Conebill (Conirostrum sitticolor), which is found at higher elevations, is much less bright red on the breast, and lacks the silvery-blue streaking on the mantle (Hilty and Brown 1986). The adult female is also easily discriminated, readily separated from other Dacnis species by the reddish-orange wash on the breast (Hilty and Brown 1986). The female could conceivably be mistaken for a female or juvenile male Crimson-breasted Finch (Rhodospingus cruentus), though the habitat preferences for these birds are very different (Ridgely and Greenfield 2001). The female Scarlet-breasted Dacnis has a yellow eye, more contrast between the brown upper and more strikingly red lower breast, and a thinner bill; whereas the female Crimson-breasted Finch has a dark reddish eye, a smoother transition from brown to duller red on the upper breast, and a thicker bill (Ridgely and Greenfield 2001).
The adult male has a forehead and mask blackish which is often not discernible (Hilty and Brown 1986, Hilty 2011). The throat, chin, jugulum, crown and nape are bright hyacinth blue (Hartert 1901, Hilty 2011). The mantle, back, and scapulars are also hyacinth blue, with bright silvery-blue streaks along the shafts (Hilty 2011). The rump is lighter and unstreaked silvery blue (Hartert 1901, Hilty 2011). The tail is black narrowly edged pale blue (Hartert 1901, Hilty 2011). The flight feathers are black, with inconspicuous dull blue edging, which is most extensive on the tertials (Hartert 1901, Hilty 2011). The back and upper wing coverts are strongly streaked light silvery blue, to a lesser extent on the crown (Hilty and Brown 1986, Hilty 2011). Across the breast is a broad band of fiery red, bleeding into yellow on the lower breast which continues to fade to whitish buff from the center of the belly to the undertail coverts (Hartert 1901, Hilty 2011). The thighs are blackish (Hartert 1901). Both adult sexes have yellow irises, blackish to black bills, and dark gray to blackish legs and feet (Hartert 1900, Hartert 1901, Hilty 2011). The female dorsally is primarily plain brown to dark sepia-brown including the wing coverts and flight feathers, brighter on the rump, with an ashy-gray tinge on the forehead (Hartert 1900, Hilty 2011). The flight feathers are narrowly edged sepia-brown outwardly and slightly grayer inwardly (Hartert 1900). The throat is light brown to a broad and diffuse fiery-red to reddish-orange wash across the breast, fading to rusty-brown flanks laterally and to rusty buff to buffy white belly and undertail coverts (Hartert 1900, Hilty and Brown 1986, Hilty 2011). The underwing coverts and axillaries are cream colored and the the tail is dark brown (Hartert 1900). Juveniles resemble the adult female but have brown instead of yellow eyes (Hilty 2011). Though adult male and female are obviously distinct, the are likely even more so to the birds themselves, for they are highly dichromatic from an avian visual perspective (Burns and Shultz 2012).
Tanagers that have been studied have either a Complex Basic Strategy or Complex Alternative Strategy (Ryder and Wolfe 2009). However, most tanagers only molt once a year, and this prebasic molt likely occurs after the breeding season (Isler and Isler 1987, Ryder and Wolfe 2009). Many species have been found to breed in subadult plumage (Isler and Isler 1987). The performative molt is partial in Dacnis (Ryder and Wolfe 2009). Although an immature plumage is described for Scarlet-breasted Dacnis (Hilty 2011), more specific information on molt and its timing is not available for this species.
For both sexes the iris is yellow in adults, brown in young birds (Hitly 2011). The bill is black to blackish, and the legs and feet are dark grey to black (Hartert 1900, Hartert 1901, Hilty 2011).
The total length is about 12 cm (Hilty and Brown 1986). Male measurements include wing: 67 mm, tail: 50 mm, mandible: 14 mm, and metatarsus: 15 mm (Hartert 1901). Reported female measurements are smaller, with wing: 61-62 mm, tail: 42 mm, mandible: 12 mm, and metatarsus: 15 mm (Hartert 1900, Hartert 1901). Mass has apparently never been reported for this species.