<b><i>Background:</i></b> Warfarin requires strict monitoring by measuring prothrombin time (PT), international normalized ratio (INR), and time in therapeutic range (TTR). Several factors can lead to poor PT/INR control including vitamin K status, medication adherence, knowledge, and quality of life. The present study aims to assess patient adherence to warfarin treatment and its correlation with INR control. <b><i>Methods:</i></b> A cross-sectional study was conducted between October 2017 and January 2018 at Tripoli University Hospital. Data were collected by structured questionnaires which included; demographic and clinical characteristics, the Oral Anticoagulation Knowledge (OAK) test, and the Morisky Medication Adherence Scale (MMAS-8). <b><i>Results:</i></b> The final analysis included 88 out of 140 patients (73.33%). There were significant differences in age range, gender, marital status, and education level between the 2 groups (poor knowledge and adequate knowledge) (<i>p</i> &#x3c; 0.05). There was a significant positive correlation between OAK test score and TTR. Overall, 76.2% of patients were adherent to warfarin (MMAS score ≥6) and 20.45% of patients were of high adherence (MMAS score of 8). The median score was 6 (IQR 6–7). A statistically significant, strong positive correlation between adherence to medication and TTR as an indicator of INR control was found (rs[86] = 0.472, <i>p</i> &#x3c; 0.0001). <b><i>Conclusion:</i></b> The study addressed and identified several areas for future improvement of patient outcomes. The implementation of new approaches to enhance patient knowledge and adherence is warranted, and measures to provide treatment for all patients that require it are needed, to improve outcomes and decrease adverse drug effects.