This study investigates the thermo-kinetic behavior and product characterization of Tamanu seed (TS) and waste polypropylene (PP) through thermogravimetric analysis (30–900 °C at 5, 10, and 20 °C/min), batch pyrolysis (400–550 °C), and pyrolytic oil analysis (Fourier transform infrared [FTIR] and gas chromatography–mass spectrometry [GC–MS]). Co-pyrolysis of TS and PP lowered the thermal degradation temperature of PP. The thermal degradation of TS exhibited the activation energy of 165 kJ mol⁻¹, following an F₂ mechanism, whereas PP showed the activation energy of 293 kJ mol⁻¹ with an F₁ mechanism. The TS–PP blend exhibited the activation energy of 280 kJ mol⁻¹ and followed a D₁ mechanism. The blend reduced the Gibbs free energy to 765.29 kJ mol⁻¹, compared to 766.87 kJ mol⁻¹ for TS and 830.46 kJ mol⁻¹ for PP alone. Co-pyrolysis improved the yield and fuel properties of the pyrolytic oil, demonstrating a sustainable and effective strategy for converting biomass and plastic waste into valuable biofuels, contributing to waste management and energy recovery.