Biodegradable poly(lactic acid) (PLA)-based composites were prepared using PLA and basalt scale (BS) via a solution-blending method. BS surfaces were treated using a silane coupling agent, and their surface properties were characterized by high-resolution scanning electron microscopy, energy-dispersive X-ray spectrometry, and X-ray photoelectron spectrometry. Moreover, the influence of BS content on the thermal properties, flexural properties, impact strength, and morphology of the PLA/silane-coupling-agent-treated BS (KH-BS) composites was analyzed. The thermal stability of the composites significantly increased due to the addition of KH-BS. Impact strength tests showed that the impact strength of the PLA/KH-BS composite with 4 wt% KH-BS was 3.14 kJ/m2, which is 51% higher than that of pristine PLA (2.07 kJ/m2). The analysis of the fracture surfaces of the composites after the impact strength tests revealed a rough morphology with numerous river-like micro-cracks. The study results demonstrate that the addition of KH-BS significantly improves the thermal stability and impact strength of PLA/KH-BS composites.