The use of medications by breastfeeding mothers requires special attention, as it can affect both maternal health and infant development. Adequate knowledge regarding medication use supports safe and appropriate practices, while limited knowledge increases the risk of adverse side effects and drug interactions. This study aimed to determine the correlation between knowledge level and medication use behavior among breastfeeding mothers in Sambirejo, Mantingan, Ngawi. The research employed a non-experimental cross-sectional design involving 33 respondents who met the inclusion criteria. Data were collected using a questionnaire that had been tested for validity (r-calculated > 0.361) and reliability (Cronbach’s Alpha > 0.6). Data analysis was performed using the Spearman rank correlation test. The results showed that 45.45% of respondents had a good level of knowledge, 48.48% had a sufficient level, and 6.06% had a poor level. Regarding medication use behavior, 51.52% of respondents were categorized as very good and 48.48% as good. Knowledge about medications among breastfeeding mothers was significantly associated with medication use behavior (p = 0.005), with a positive moderate correlation (r = 0.476). This indicates that higher levels of knowledge are linked to better medication use behavior. In conclusion, there is a significant correlation between knowledge level and medication use behavior among breastfeeding mothers in Sambirejo.