Recycled polyethylene terephthalate (rPET) is a promising but underused feedstock for fused deposition modeling (FDM), since thermal and hydrolytic degradation during recycling lowers its molecular weight, causes embrittlement, and weakens interlayer bonding. Although waste cooking oil (WCO) has been explored as a bio-based plasticizer in other polymer systems, this study addresses the largely unreported use of chemically revised WCO (rWCO) as plasticizer for rPET filaments in FDM, offering chemical, thermal, mechanical, and microstructural evidence of its compatibility and benefits within this system. The WCO was chemically revised through a cornstarch pre-treatment followed by methanol/NaOH-mediated esterification and transesterification (200:50:50 mL ratio, 2 h stirring, 7-day phase separation). Fourier-transform infrared (FTIR) analysis confirmed a progressive decrease in free fatty acid content and moisture across the cornstarch, transesterification, and glycerol-separation stages, verifying successful oil revision prior to blending. rWCO was incorporated into rPET pellets at 0, 6.5, 7.5, and 8.5 wt.% before twin-screw extrusion into filament, and FDM-printed dog-bone specimens were assessed via differential scanning calorimetry (DSC) and tensile testing (n = 3 per formulation, mean ± standard deviation). Tensile strength dropped from 14.10 ± 0.43 MPa (neat rPET) to 10.45–11.52 MPa with rWCO addition, with the 7.5 wt.% blend retaining the highest strength (11.52 ± 0.30 MPa) and stiffness among modified samples. SEM of the 6.5 and 7.5 wt.% fracture surfaces linked mechanical behavior to filament morphology and interlayer bonding. Overall, rWCO reduces rPET's glass transition temperature and enhances processability, with 7.5 wt.% offering the best balance of strength retention and ductility.