To compare two health literacy measurements' ability to assess older adults' medication adherence by using the Korean Health Literacy Screening Questions (KHLSQ) and the Modified Korean Functional Health Literacy Test (M-KFHLT), and to identify an appropriate health literacy measurement. Lower health literacy has been associated with poorer medication adherence. Thus, health professionals should evaluate the available health literacy assessment instruments they are using and choose an appropriate instrument to assess health literacy to increase older adults' medication adherence. Following the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) checklist, a descriptive, cross-sectional study was conducted. From November 2017-May 2018, 116 community-dwelling older adults were recruited in South Korea. Questionnaires were completed during a face-to-face interview with each participant in a private room; health literacy was assessed using the KHLSQ and the M-KFHLT and medication adherence using the Morisky Medication Adherence Scale. Health literacy assessed using the KHLSQ was found to be a predictor of medication adherence, but was not a predictor when measured by the M-KFHLT. Having low income, multiple chronic diseases and vision problems were also significant factors related to medication adherence. This study suggests that health literacy was negatively associated with medication adherence. Furthermore, KHLSQ is an appropriate tool for healthcare providers to use when assessing health literacy to predict older adults' medication adherence. This finding indicated that healthcare providers should select an appropriate health literacy measurement that suits their purposes and the population they serve, particularly for older adults.