Weed seeds with mechanical damage are more susceptible to mortality in soil than non-damaged seeds. In this study we introduce a colorimetric assay to distinguish mechanically damaged weed seeds from non-damaged weed seeds. Our objectives were to (1) compare steepates from mechanically damaged seeds against steepates from non-damaged seeds for their capacities to reduce resazurin — a non-toxic, water soluble dye that changes color and light absorbance properties in response to pH, and (2) use light absorbance data from steepate-resazurin solutions to create classification trees for distinguishing damaged from non-damaged weed seeds. Species in this study included barnyardgrass, curly dock, junglerice, kochia, oakleaf datura, Palmer amaranth, spurred anoda, stinkgrass, tall morningglory and yellow foxtail. Seeds of each species were subjected to mechanical damage treatments that collectively represented a range of damage severities. Damaged and non-damaged seeds were individually soaked in water to produce steepates that were combined with resazurin. Light absorbance properties of steepate-resazurin solutions indicated that, for all species except kochia, damaged seeds reduced resazurin to greater extents than non-damaged seeds. Prediction accuracy rates for classification trees that used absorbance values as predictor variables were conditioned by species and damage type. Prediction accuracy rates were relatively low (66 to 86% accurate) for lightly damaged seeds, especially grass weed seeds. Prediction accuracy rates were high (91 to 99% accurate) for severely damaged seeds of specific broadleaf and grass weeds. Steepate-resazurin solutions that successfully separated seeds took no more than 32 h to produce. The results of this study indicate that the resazurin assay is a method for quickly distinguishing damaged from non-damaged weed seeds. Because rapid assessments of seed intactness may accelerate the development of tactics for reducing the number of weed seeds in soil, we advocate further development of resazurin assays by laboratories studying methods for weed seedbank depletion.