During citizen-science expeditions to the Ulu Temburong National Park, Brunei, several individuals were collected of a semi-slug species of the genus Microparmarion that, based on morphology and in-the-field DNA-barcoding, was found to be an undescribed species. In this paper, we describe Microparmarion sallehi Wu, Ezzwan & Hamdani, n. sp., after field centre supervisor Md Salleh Abdullah Bat. We provide details on the external and internal reproductive morphology, the shell and the ecology of the type locality, as well as a diagnosis comparing it with related species. DNA barcodes were generated for five individuals and used for a phylogenetic reconstruction. Microparmarion sallehi sp. n. and M. exquadratus Schilthuizen et al., 2019 so far are the only Bornean species of the genus that live in lowland forest; other species are found in montane forests.