Biodegradable polymers are essential for sustainable plastic life cycles and contribute to a carbon-neutral society. Here, we explore the development of biodegradable fibers with excellent mechanical properties using polypropylene (PP) and thermoplastic starch (TPS) blends. To address the inherent immiscibility between hydrophobic PP and hydrophilic TPS, hydrophilic modification and a masterbatch approach were employed. Melt-spinning trials demonstrated that the modified PP and TPS blends (mPP/TPS) exhibited excellent spinnability and processability comparable to virgin PP. A sheath-core configuration was introduced to enhance biodegradability while maintaining structural stability, with an mPP-rich part as the core and a TPS-rich part with a biodegradable promoter (BP) as the sheath. SEM and DSC analyses confirmed strong interfacial compatibility, uniform fiber morphology, and single melting points, indicating no phase separation. Mechanical testing showed that the sheath-core fibers met industrial requirements, achieving a tenacity of up to 2.47 gf/den and tensile strain above 73%. The addition of a BP increased the biodegradability rate, with PP/TPS/BP fibers achieving 65.93% biodegradation after 115 days, compared to 37.00% for BP-free fibers. These results demonstrate the feasibility of blending petroleum-based polymers with bio-based components to create fibers that balance biodegradability, spinnability, and mechanical performance, offering a sustainable solution for industrial applications.