Ultrathin two-dimensional (2D) semiconductors exhibit outstanding properties, but it remains challenging to obtain monolayer-structured inorganic semiconductors naturally occurring as nonlayered crystals. Copper sulfides are a class of widely studied nonlayered metal chalcogenide semiconductors. Although 2D copper sulfides can provide extraordinary physical and chemical applications, investigations of 2D copper sulfides in the extreme quantum limit are constrained by the difficulty in preparing monolayered copper sulfides. Here, we report a subnanometer-thin quasi-copper-sulfide (q-CS) semiconductor formed upon self-assembly of copper(I)-dodecanethiol complexes. Extended X-ray absorption fine structure analysis revealed that the existence of Cu-Cu bonding endowed the layer-structured q-CS with semiconductor properties, such as appreciable interband photoluminescence. Theoretical studies on the band structure demonstrated that the optical properties of copper-dodecanethiol assemblies were dominated by the q-CS layer and the photoluminescence originated from exciton radiative recombination across an indirect band gap, borne out by experimental observation at higher temperatures, but with the onset of a direct emission process at cryogenic temperatures. The following studies revealed that the metal-metal bonding occurred not only in copper-alkanethiolate complex assemblies with variable alkyl chain length but also in silver-alkanethiolate and cadmium-alkanethiolate assemblies. Therefore, the current studies may herald a class of 2D semiconductors with extremely thin thickness out of nonlayered metal sulfides to bridge the gap between conventional inorganic semiconductors and organic semiconductors.