Three-branched bamboo is a taxonomically difficult group due to recent rapid radiation and frequent hybridization, with generic and species delimitation of many taxa problematic. Indosasa solearis, described from North Vietnam based on flowering materials, was later transferred to Sinobambusa. But its taxonomic position remains arguable due to limited vegetative data. After a detailed morphological study of this bamboo at its type locality, we found that the overall morphology of this species is surprisingly similar to I. parvifolia and I. jinpingensis. Thus, we conducted morphological and phylogenomic analyses to ascertain the taxonomic position of I. solearis and its relationship with those two species. Morphological examination confirmed I. solearis exhibits diagnostic characters of Indosasa, like leptomorph rhizome, diffuse culms, strongly and asymmetrically raised branch supra-nodal ridges, 4–9 foliage leaves per ultimate branch, pseudospikelet, and each floret with six stamens and three stigmas. No essential difference was found among the three Indosasa species both on macro- and micromorphology. Phylogenies based on single-copy orthologous nuclear gene datasets and single nucleotide polymorphism data strongly support I. solearis as an Indosasa member, closely related to I. parvifolia and I. jinpingensis. Population genetic analyses revealed identical genetic component across all the three species. Combined with morphological and molecular evidence, we confirmed that I. solearis belongs to Indosasa, and is conspecific with I. parvifolia and I. jinpingensis. We therefore reinstate I. solearis and synonymize I. parvifolia and I. jinpingensis with it. Furthermore, our analyses also revealed the hybrid origin of Sinobambusa tootsik, with genomic signatures suggesting an ancient cross between Indosasa and Pleioblastus. This finding highlights the reticulate evolutionary history in the three-branched bamboo.