The current research was conducted to evaluate the general properties of recycled polyethylene terephthalate (rPET) composites reinforced with glass fiber (GF) in terms of their potential suitability for engineering applications. The test samples were produced by injection molding method. With process parameters properly set, this method allows the production of well-densified materials with a negligible amount of voids. Composites of various glass fiber contents (0, 20, 30, 40, and 50% by mass) were analyzed by performing mechanical, thermal, and morphological studies, and the results of the tensile and flexural tests are herein presented in detail. The research results showed a significant improvement in the mechanical strength and thermal stability of glass fiber-reinforced composites compared to unreinforced rPET. The analysis of the literature shows that comparable strength values were obtained for non-recycling materials, especially from the group of polymers. The morphological analysis allowed for understanding changes in the internal structure of the composites, which had a significant impact on their mechanical properties. It has been proven that the produced composites showed adequate dispersion and distribution of GF in the PET matrix and high reinforcement effectiveness. The conclusions from the conducted research may contribute to the further development of technologies using recycled materials to improve the efficiency and sustainability of production processes in the field of engineering, in particular land, transport, mining, and maritime.