Despite the popularity of fiber reinforced polypropylene (PP) composites both in academia and industry, the research on basalt fiber (BF) reinforced thermoplastic PP composites is quite limited. Due to their superior properties, BFs have a great commercial potential for industrial applications. In this study, six different silane coupled BF/PP composites were prepared with fiber contents up to 15 wt% using a twin-screw extruder. Maleic anhydride grafted PP (MAH-g-PP) coupling agent content was fixed at 10 wt%. Mechanical properties were investigated by tensile, flexural, and Izod impact tests. Thermal, viscoelastic, and morphological properties were studied by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and scanning electron microscopy (SEM), respectively. The results showed that BF has substantial effect on mechanical and viscoelastic properties of the composites. The considerable increase in tensile strength, flexural strength, and Izod impact was attributed to the higher interfacial adhesion between the BF and PP matrix in the presence of MAH-g-PP coupling agent. The SEM images proved the increased interfacial adhesion between the fibers and the PP matrix. The improved interfacial adhesion was also affected the crystallization behavior of PP.