Background: Fixed orthodontic treatment is widely used to correct malocclusions; however, its early phase often involves discomfort and dietary changes that may affect patients’ nutritional status. These factors can lead to temporary variations in body mass index (BMI). This study aimed to evaluate the impact of fixed orthodontic treatment on BMI during the early months of therapy. Methods: A prospective observational study was conducted in the Department of Orthodontics, Bangabandhu Sheikh Mujib Medical University (BSMMU), Dhaka, from October 2022 to September 2023. A total of 130 patients aged ≥18 years undergoing fixed orthodontic treatment were selected by simple random sampling. Body weight and height were measured, and BMI was calculated at baseline (T1), one month (T2), and three months (T3) after appliance placement. Data were analyzed using paired and unpaired t-tests, with p < 0.05 considered significant. Results: Participants had a mean age of 21.26 ± 2.40 years, and females comprised 59.2%. At baseline, 68.5% had a normal BMI. Mean BMI decreased significantly from 21.92 ± 2.93 at T1 to 21.74 ± 3.02 at T2 (p < 0.001), then increased to 21.95 ± 2.96 at T3 (p < 0.001 vs. T2; p = 0.34 vs. T1). BMI decreased in 53.1% of participants at one month but increased in 74.6% between one and three months. No significant sex differences were observed (p > 0.05). Conclusion: Fixed orthodontic treatment caused a temporary decline in BMI during the first month, followed by recovery to baseline levels by three months. Early nutritional monitoring and dietary counseling are recommended during the initial treatment phase.