Tempeh fermentation, a traditional Indonesian food processing method, has been recognized for enhancing the nutritional, bioactive, and sensory properties of soybeans through fermentation with the fungus Rhizopus oligosporus. Although traditionally made from soybeans, this study explores using black beans as a novel ingredient for tempeh fermentation. Black beans, which are rich in protein and bioactive compounds like flavonoids and phenolics, have a unique nutritional profile that has potential for developing into functional, nutritious, plant-based food products. This study evaluates the effects of tempeh fermentation on black beans by comparing their nutritional composition, phenolic content, flavonoid content, antioxidant activity, and antinutrient levels using proximate analysis, colorimetric assays, and in vitro assays before and after 40 hours of fermentation. In addition, sensory analysis was also conducted to test the consumer acceptance of the black bean tempeh, by comparing it to the traditional and commercial soybean tempeh. The results show that tempeh fermentation significantly increased the water content, protein content, total carbohydrate, reducing sugar, free phenolics, flavonoids, bound phenolics, and antioxidant activity. On the other hand, it significantly reduced the fat content, trypsin inhibitors, and phytic acid levels of black beans. Sensory results showed that black bean tempeh achieved comparable hedonic liking scores to commercial soy tempeh in taste, flavor, and texture, though scored significantly lower than traditional soy tempeh, particularly in appearance. Bitterness, nutty flavor, and umami taste were also found to be the drivers of overall liking. These findings demonstrate that fermenting black beans into tempeh produced an affordable, nutritious, and functional plant-based protein with potential for consumer acceptability in the growing plant-based market.