In this work, a heterogeneous catalyst based on palladium nanoparticles encapsulated on polyamidoamine dendrimer-grafted multi-walled carbon nanotubes (PdNPs/PAMAM–MWCNTs) hybrid material was used as an electrode modifier. The catalyst was dissolved in N,N-Dimethylformamide (DMF) and immobilized on the surface of glassy carbon electrode using drop casting method to fabricate PdNPs/PAMAM–MWCNTs modified electrode. The modified electrode showed good catalytic activity for the electrooxidation of oxalic acid in 0.1 M HClO₄ with a substantial decrease in anodic overpotentials (450 mV) and increase in anodic peak current (about 3 times) in comparison with unmodified electrode. The values of electron transfer coefficients (α = 0.37) and diffusion coefficient (D = 2.12 × 10⁻⁶ cm² s⁻¹) were calculated for oxalic acid using electrochemical approaches. The cyclic voltammetry showed a linear response in the range of 0.03–5.0 mM with detection limit (3σ) of 0.02 mM for oxalic acid. The experimental results also showed that the sensor has good reproducibility (R.S.D = 4.3% for 5.0 mM, n = 5), long-term stability (over than 100 analysis with intermittent use), and a fast response time (5 s). Finally, the modified electrode was successfully applied to determine the oxalic acid content in spinach.