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Boron-based two-dimensional (2D) materials are an excellent platform for nanoelectronics applications. Rhombohedral boron monosulfide (r-BS) is attracting particular attention because of its unique layered crystal structure suitable for exploring various functional properties originating in the 2D nature. However, studies to elucidate its fundamental electronic states have been largely limited because only tiny powdered crystals were available, hindering a precise investigation by spectroscopy such as angle-resolved photoemission spectroscopy (ARPES). Here we report the direct mapping of the band structure with a tiny (∼20 × 20 μm2) r-BS powder crystal by utilizing microfocused ARPES. We found that r-BS is a p-type semiconductor with a band gap of >0.5 eV characterized by the anisotropic in-plane effective mass. The present results demonstrate the high applicability of micro-ARPES to tiny powder crystals and widen an opportunity to access the yet-unexplored electronic states of various novel materials.