Objectives Hypothetical models of socioeconomic organization in pre‐Columbian societies generated from the rich ethnohistoric record in the New World require testing against the archaeological and bioarchaeological record. Here, we test ethnohistorian Maria Rostworowski's horizontality model of socioeconomic specialization for the Central Andean coast by reconstructing dietary practices in the Late Intermediate Period (c. AD 900‐1470) Ychsma polity to evaluate complexities in social behaviors prior to Inka imperial influence. Materials and methods Stable carbon and nitrogen isotope analysis of archaeological human bone collagen and apatite (δ13Ccol[VPDB], δ15Ncol[AIR], δ13Cap[VPDB]) and locally available foods is used to reconstruct the diets of individuals from Armatambo (n = 67), associated ethnohistorically with fishing, and Rinconada Alta (n = 46), associated ethnohistorically with agriculture. Results Overall, mean δ15Ncol[AIR] is significantly greater at Armatambo, while mean δ13Ccol[VPDB] and mean δ13Cap[VPDB] are not significantly different between the two sites. Within large‐scale trends, adult mean δ13Cap[VPDB] is significantly greater at Armatambo. In addition, nearly one‐third of Armatambo adults and adolescents show divergent δ15Ncol[AIR] values. Discussion These results indicate greater reliance on marine resources at Armatambo versus Rinconada Alta, supporting the ethnohistoric model of socioeconomic specialization for the Central Andean coast. Deviations from large‐scale dietary trends suggest complexities not accounted for by the ethnohistoric model, including intra‐community subsistence specialization and/or variation in resource access.