The objective of this study was to investigate the utilization of tea mill waste fibers (TMWF) in thermoplastic composites. For this purpose, two plastic types: polypropylene (PP) and high density polyethylene (HDPE), were used as thermoplastic material while TMWF was utilized as a lignocellulosic filler. HDPE or PP and up to 50% TMWF were compounded in the single screw extruder and extrudates were compression molded into thermoplastic composite panels. The effects of plastic type and fiber loading rate on mechanical (flexural, tensile, and impact properties), physical (water absorption and thickness swelling), thermal properties, and morphological of the produced composites were determined. Tensile modulus, flexural modulus, water absorption, and thickness swelling of the produced composites were increased with the rise of the TMWF amount in the thermoplastic matrix. On the other hand, TMWF increase in the thermoplastic matrix reduced the tensile strength, flexural strength, and impact strength of the produced composites. It should also be noted that both flexural strength and flexural modulus have satisfied the requirements of ASTM D 6662. In the case of thermal properties, addition of TMWF into the thermoplastic matrix did not change the initial degradation significantly. However, the char rate of the composites increased. It appears that tea mill waste fibers may have a potential usage as filler in the PP- and HDPE-based thermoplastic composites.