This article provides a comprehensive quantitative analysis of misspellings in the writing of fluent Spanish heritage language (SHL) learners enrolled in a university-level Spanish course. The corpus consisted of a total of 21,322 words and 2492 misspellings, which were divided into four main categories and several subcategories. This paper details the misspelling patterns found in the data and argues that they should guide a pedagogical intervention designed specifically for these learners. These students demonstrated a good command of one-to-one grapho-phonemic relationships in Spanish, but also a need to develop knowledge of inconsistent grapho-phonemic relationships and written accent marks. For both consonants and vowels, interference from English spelling also occurred. This study, as the first of its kind, aims to begin filling the gap in the literature on spelling research in the SHL context in particular and to contribute to our understanding of spelling development in general. (Contains 8 tables.)