Two subspecies are recognized for Dacnis venusta (Dickinson and Christidis 2014, Clements et al. 2015).

venusta, described as Dacnis venusta Lawrence 1862
Type Locality: Panama Railroad, Panama
It is distributed in the tropical zone on both slopes in Costa Rica and western Panama near Chiriqui (Storer 1970, Clements et al. 2015). See Detailed Description.

fuliginata, described as Dacnis venusta fuliginata Bangs, 1908 Proc. Biol. Soc. Washington 21: 160
Type Locality: Jiménez, western Colombia
This subspecies is endemic to the Chocó and Nechí Centers of Endemism (Cracraft 1985). It is fairly common in Panama in the lowlands and foothills of eastern Caribbean slope of San Blas and Darién, through western Colombia, to northwestern Ecuador (Storer 1970, Wetmore et al. 1984, Clements et al. 2015). It is similar to nominate venusta, but has deeper sooty black underparts and lacks conspicuous green hues, or the greenish tinge is restricted to the belly (Hellmayr 1911, Wetmore et al. 1984). This subspecies is smaller than the Central American nominate subspecies, especially in bill length, alhough measurements are more similar where these subspecies meet in southern Panama (Hellmayr 1911). Measurements for this subspecies are reported in Table 1 below, and measurements for the nominate subspecies are reported in the table in Measurements.

Scarlet-thighed Dacnis is one of nine members of the genus Dacnis (Dickinson and Christidis 2014, Clements et al. 2015). Molecular phylogenetics have recovered Dacnis in a clade with Cyanerpes and Tersina, with strong support, and these lineages have been termed the Dacninae, or the Blue Tanagers (Burns et al. 2014, Burns et al. 2016). How these three genera are related within the Dacninae currently is unresolved, however (Burns et al. 2003, Burns et al. 2014). Though the three genera differ markedly in the bill morphology, they share many characters, such as adult males that usually are bright blue and heavily reflecting UV light, and females usually are greenish (Burns and Shultz 2012, Burns et al 2014). Though the genus Dacnis was recovered as monophyletic, with strong support, more relationships among the species within Dacnis are not resolved. Scarlet-thighed Dacnis was recovered as sister to all other Dacnis species with high support, although Scarlet-breasted Dacnis (Dacnis berlepschi) was not available for inclusion in this analyses (Burns et al. 2014). The genus Dacnis formerly was placed in the New World honeycreeper family Coerebidae (e.g., Boucard 1878, Sclater 1886, Bangs 1907, Hellmayr 1935). Some research uncovered similar morphological features of some "coerebids" with tanagers, such as horny palates, skull morphology, jaw muscles, and plumage similarities (Beecher 1951, Tordoff 1954, Storer 1969). All species of New World honeycreepers were placed in the Thraupidae by Storer (1970), and this classification has been subsequently supported by molecular phylogenetics (Burns et al. 2003, Burns et al. 2014); consequently, Coerebidae no longer is recognized as a family.