This study revealed the behaviour of the effect of varying the methylene diphenyl diisocyanate (MDI) index on foaming reactivity, physico-mechanical and morphological properties of palm olein-based viscoelastic polyurethane (VE-PU) foams. The palm olein-based polyol, E-135 NOP was used from 15% to 25% (w/w) in the VE-PU foam formulations. Our study revealed that by varying the isocyanate indices, no significant changes in cream and gelling times were observed, but the rise time was slightly decreased, indicating that the rate of reaction had increased. The compression force deflection (CFD) and tensile strength with palm oleinbased VE-PU foam with 25% of E-135 NOP were significantly increased as the isocyanate indices increased. However, it was observed that other mechanical properties, i.e., density and resiliency of palm olein-based VE-PU foam decreased as the isocyanate indices increased. In terms of morphology, the average cell size determined by scanning electron microscope (SEM) also increased as the isocyanate indices increased. The finding of this study could have practical significance in the selection of the optimal isocyanate index to formulate VE-PU products.