This study aimed to develop a diagnostic instrument to assess children’s developmental level of spelling abilities, which can be used to identify students with writing disability. Based on a rigorous review of previous research and three pilot studies, the Korean Developmental Spelling Assessment (KDSA) was developed. The KDSA comprised of two spelling tests: the phonological feature test and the morphological feature test. The phonological feature test assessed students’ abilities in spelling five phonological features: liaison, initial consonants/principal vowels, aspirated/ tense consonants, final consonants, and w/y glides. Moreover, the morphological spelling feature test assessed students’ abilities in spelling six morphological features; simplification of final consonants, co-articulated tensification, co-articulated aspiration, consonant assimilation, compound consonants, and compound vowels. The KDSA was standardized with 1,049 students across the county. The results of point-biserial correlation, polychoric correlation, Cronbach’s alpha coefficient, and split-half correlation revealed that the KDSA is a highly reliable measure. The limitations of this study and suggestions for future research were discussed.