This study describes the development of a Korean assessment instrument measuring adults' basic literacy ability. Instrument items were generated from a literature review, inputs from experts through the Delphi method, and focus group interviews. Items and measurement procedures were tested and confirmed through three steps of pilot implementations. As a result, three sets of test items measuring up to level 4 of the Korean basic literacy ability for adults were successfully developed. This assessment is simple, fast, and inexpensive and reflects the distinctive characteristics of the Korean language and Korean contexts. The instrument's practicality will contribute to its further utilization in identifying and supporting adults in need of basic literacy education.