This paper presents the adaptation of the POMAS classification of spelling errors (Silliman et al., Developmental Neuropsychology 29:93-123, 2006, Bahr et al., Journal of Speech, Language & Hearing Research 55:1587-1599, 2012; International Journal of Bilingual Education and Bilingualism 18:73-91, 2015) to Polish orthography. We identified the most common phonological, orthographic, and/or morphological errors committed by Polish Grade 1 spellers based on a writing-to-dictation task. Some of the error types were included in the original POMAS; some we added, following the classification rationale, to adapt the classification to the specific features of the Polish orthography. Moreover, we found that the participants made, on average, an equal number of phonological and orthographic errors, and of phonological and morphological errors. However, orthographic errors were more frequent than morphological ones. A roughly equal proportion of orthographic and phonological errors occurred in the spelling of the most difficult words. The most frequent orthographic error type was grapheme substitution, followed by final consonant devoicing, and consonant devoicing within a word errors. The most frequent phonological error type were misspellings in diacritic signs, followed by consonant deletion, vowel deletion, and epenthesis. The developed POMAS-PL version may be used by other researchers of Slavic languages, to allow for intercultural comparisons. Moreover, our analyses can be used by teachers of Polish as a first and a second language to anticipate their students' errors and to understand what knowledge the learners need to learn to spell correctly.