Part of a species complex that also includes Taiwan Grasshopper-warbler (Locustella alishanensis), Russet Grasshopper-warbler (Locustella mandelli), Brown Grasshopper-warbler (Locustella luteoventris), Benguet Grasshopper-warbler (Locustella seebohmi) and Timor Bush-Warbler (Locustella timorensis) and some or all are often treated as conspecific. Close to Benguet Grasshopper-warbler (Locustella seebohmi) and Timor Bush-Warbler (Locustella timorensis), but most similar to Russet Grasshopper-warbler (Locustella mandelli). Birds on Bali presumed to be of this species, very similar in plumage and structure to those on Java but exhibit some rather subtle vocal differences from latter; identity uncertain, but may prove to represent an undescribed subspecies of present species. Further study is required.
The following 2 subspecies are recognised:
montis (E. Hartert, 1896) - Java, Timor.
timorensis (Mayr, 1944) - Mountains of Timor and Alor (eastern Lesser Sundas). Considered by some authors to be a distinct species, Timor Bush-Warbler (Locustella timorensis).