ABSTRACT Sometime in the early 17th century, at Magdalena de Cao, a community of resettled native peoples in the Chicama Valley on the North Coast of Peru, a Spaniard used the back of a letter to jot down the terms for numbers in a local language. Four hundred years later, the authors of this article were able to recover and study this piece of paper. We present information on this otherwise unknown language, on numeracy, and on cultural relations of ethnolinguistic groups in pre‐ and early‐post‐Conquest northern Peru. Our investigations have determined that, while several of the Magadalena number terms were likely borrowed from a Quechuan language, the remainder record a decimal number system in an otherwise unknown language. Historical sources of the region mention at least two potential candidate languages, Pescadora and Quingnam; however, because neither is documented beyond its name, a definite connection remains impossible to establish.