Centaurium japonicum (Maxim.) Druce is a synonym of Schenkia japonica (Maxim.) G.Mans.

The former name was transferred to Schenkia by Mansion (2004), who provided the phylogenetic relationships among genera of the subtribe Chironiinae (Gentianaceae), though no detailed phylogeny about each species was shown.
Later, in Mansion & Struwe (2004), a detailed version was shown but not included Schenkia japonica. However, based on the key in Mansion (2004), this species is morphologically fit to the genus Schenkia.

A1. Style slightly bifid; stigma lobes reniform to shoe-shaped, fleshy; capsule linear in shape . . . . . . . . . . . . . . . . . . . . . . .Centaurium Hill
A2. Style not divided to subcapitate; stigma lobes rhom-bic to fan-shaped, not fleshy; capsule elliptic to ovalin shape .. . . . . . . . . . . . . . . . .B.
B1. Inflorescence a spiciform cyme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Schenkia Griseb.
B2. Inflorescence a corymbiform or paniculate cyme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Zeltnera

POWO adopted Mansion (2004)'s treatment:
https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:366661-1

Mansion, G. (2004). A new classification of the polyphyletic genus Centaurium Hill (Chironiinae, Gentianaceae): description of the New World endemic Zeltnera, and reinstatement of Gyrandra Griseb. and Schenkia Griseb. Taxon, 53(3), 719-740.
https://doi.org/10.2307/4135447

Mansion, G., & Struwe, L. (2004). Generic delimitation and phylogenetic relationships within the subtribe Chironiinae (Chironieae: Gentianaceae), with special reference to Centaurium: evidence from nrDNA and cpDNA sequences. Molecular Phylogenetics and Evolution, 32(3), 951-977.
https://doi.org/10.1016/j.ympev.2004.03.016