BackgroundOver the past 20 years, there has been an increase in demand for complete knee replacements, and this trend is predicted to continue. It has been shown that being overweight is a risk factor for knee osteoarthritis. There are only a few studies on this in India and none on South Indian patients, Therefore, our goal was to evaluate how BMI affected functional outcomes after primary total knee replacement. ObjectivesTo determine the impact of body mass index (BMI) on functional outcomes after primary total knee replacement. Methodology: Patients who underwent total knee replacement between November 2021 and November 2023 were included in the study. Patients were divided into groups based on BMI. Group I patients have a BMI less than 25, and group II patients of BMI greater than 25. International Knee Society scoring(IKSS) is used to assess patients Preoperatively and postoperatively. ResultsOut of 185 patients, 70 were males and 115 were females. When IKSS scores were analyzed the mean Knee score before surgery in Group 1 was 24.58 and in Group 2 it was 16.64. After 1 year follow up the mean scores were 68.5 and 57.5 respectively. When analyzed with functional score the pre-op scores for groups 1 and 2 were 32.58 and 23.44 respectively and post-op scores after one-year follow-up were 71.17 and 51.7 respectively. ConclusionBMI does have a positive correlation with both preoperative and postoperative scores. A weight-loss programme can be discussed with the patients presenting the results of this study.