The sagebrush steppe is a patchwork of species and subspecies occupying distinct environmental niches across the intermountain regions of western North America. These ecosystems face degradation from disturbances and exotic weeds. Using sagebrush seed that is matched to its appropriate niche is a critical component to successful restoration, improving habitat for the threatened greater sagegrouse and other species. The need for restoration is greatest in basin habitats composed of two subspecies: diploid basin big sagebrush (A. tridentata subsp. tridentata) and tetraploid Wyoming big sagebrush (subsp. wyomingensis). In this study we assess seed weights across five subspecies-cytotype groups of big sagebrush and examine the genetic and environmental components. Our goal is to determine if seed weight can be used as a diagnostic test for subspecies and seed certification. Seed weight was measured from 55 wild collections and from progeny derived from these collections and grown in two common gardens. A linear mixed-effect model showed 91% of the variation in seed weight is explained by genetic, genetic 3 environment and environmental effects (conditional R2 ¼ 0.91). Moreover, genetic effects alone, subspecies-cytotype groups, explained 39% of the variation (marginal R2 ¼ 0.39). Of the five subspeciescytotype groups, most had overlapping weights using conservative 99% confidence intervals. However, diploid tridentata and wyomingensis had non-overlapping 99% confidence intervals. To demonstrate the application of seed weighing to assess the subspecies purity of commercial seed lots, we compared confidence intervals of tridentata and wyomingensis developed from the experimental data to seed weights of commercial lots. The results showed that only 17% of the commercial seed lots certified as wyomingensis had mean seed weights that fell within the confidence intervals for this subspecies. The remaining lighter seed lots (83%) matched weights of tridentata. While restoring sagebrush ecosystems is a multifaceted problem, a fundamental component to restoration is ensuring the appropriate seed is used. We found seed weight is principally affected by genetic factors, with limited environmental effects. Seed weighing is an effective application to assess subspecies purity of wyomingensis and tridentata seed and could be used as a certification step for evaluating commercial collections used in restoration.