The effective diffusion coefficient of water in marshmallow (Dwm) was determined by using a combined experimental-computational approach. Marshmallow samples were placed in a desiccator, and sample weights were recorded until there was minimal change (<0.2%, dry basis) over a period of twelve hours. A finite-difference model, accounting for diffusive and convective mass transfer, was developed to simulate the drying experiment. By minimizing the root-mean-square error between the experimental and simulation data, Dwm was determined to be 9.49 x 10⁻¹¹± 0.56 x 10⁻¹¹ m²/s. Results also indicated that the experimental drying process is limited by internal diffusion (Bi > 1).