This study explores the impact of blending polyethylene terephthalate (PET) with polybutylene terephthalate (PBT) on the thermal, structural, and mechanical properties of 3D-printed materials. Comprehensive analyses, including Fourier-transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and mechanical testing, were conducted to assess the influence of blend composition. FT-IR confirmed that PET and PBT blend physically without transesterification, while TGA showed enhanced thermal stability with increasing PET content. XRD revealed that PET and PBT crystallize separately, with the crystallinity decreasing sharply for blends with more than 50% PET. The DSC results indicated that PET effectively slows down the crystallization kinetics of PBT, promoting cold crystallization. Mechanical tests demonstrated that the elastic modulus remains relatively unchanged, but the strain at break decreases with a higher PET content, indicating increased stiffness and reduced ductility. Overall, incorporating PET into PBT improves 3D-printability and dimensional stability, reducing warpage and enhancing print precision, making these blends advantageous for 3D-printing applications.