Water diffusion testing is typically carried out by immersing specimens in a water bath and monitoring water uptake until saturation is reached. Determination of diffusivity may require several months and even years for thick specimens. In this paper, we present a water droplet-based method for rapid characterization of diffusivity. The method involves placement of a water droplet on a flat surface of the testing material. A tensiometer is used to monitor and record the evaluation of droplet dimensions. The small volume of the water droplet (below 10 μL) ensures that diffusivity can be determined in a couple of hours. The capability of this method is demonstrated by determining the water diffusion (D) of polymethylmethacrylate (PMMA) and epoxy plastics. The water diffusivity measured for PMMA matched well with published results. The droplet method was also applied to void-free epoxy and epoxy with a range of void contents. The diffusivity for the epoxy with voids increased with increasing void content. The diffusivity results for the epoxy without voids and with small void content agree with those determined from the long-term water immersion method. For the high-void-content epoxy, the diffusivity was much higher than that in the immersion method. This may be because of the rough surface caused by large exposed voids.