Empirically-based economic injury levels are lacking for pea aphid, Acyrthosiphon pisum Harris (Hemiptera: Aphididae), as a direct pest of dry peas, Pisum sativum Linnaeus (Fabaceae). To address this need, the relationship between pea aphid density and yield of dry pea (cultivar Aragorn) were quantified by encaging pea aphids at varying densities for 17-day infestation periods during 2009 and 2010 in Moscow, Idaho, United States of America. Pea aphid density after infestation at the early reproductive stage of the crop (x) significantly reduced dry pea seed yield (relative weight of US #1 dry peas, y): y = 0.7733 − 0.00998x + 0.000037x2. Economic injury levels were computed based on this relationship and incorporating the cost of control, crop market value, insecticide efficacy, and crop yield potential. The resulting economic injury levels ranged from five to 19 pea aphids per plant at the start of early reproductive growth stages of dry peas. For usability these were converted to sweep net sample size equivalents of 86–307 pea aphids per twenty-five 180-degree sweeps with a standard sweep net. These economic injury levels are applicable in the inland Pacific Northwest, United States of America, where they were developed and likely in other regions with similar climatic and agronomic conditions.