Background: Osteoporosis is a major public health concern among postmenopausal women, leading to increased fracture risk and morbidity. Bone mineral density (BMD) assessment is the most reliable method for early detection, with Dual-Energy X-ray Absorptiometry (DEXA) considered the gold standard. Identifying factors associated with reduced BMD is essential for timely preventive interventions, especially in populations with limited healthcare access. Methods: This cross-sectional study was conducted in the Department of Orthopedics, District Sadar Hospital, Rajbari, Bangladesh, from February 2020 to January 2022. A total of 300 postmenopausal women aged ≥45 years, with spontaneous cessation of menstruation for at least one year, were included through purposive sampling. Demographic and clinical data were collected and BMD was measured at the lumbar spine and femoral neck using DEXA. Data were analyzed using SPSS version 25, with Chi-square tests to assess associations between BMD and age, duration since menopause and body mass index (BMI). Results: Among 300 participants, the mean age was 58.6 ± 7.4 years and the mean duration since menopause was 9.2 ± 5.8 years. Osteopenia was most prevalent (45.7%), followed by normal BMD (28.3%) and osteoporosis (26%). Osteoporosis prevalence increased with age (from 9.2% in 45–50 years to 52.9% in >65 years, p < 0.001) and longer menopausal duration (10.4% in <5 years to 40.7% in >15 years, p = 0.002). Underweight women had the highest prevalence of osteoporosis (48.8%), while overweight and obese women had higher rates of normal BMD (38.9% and 38.5%, respectively, p = 0.015). Conclusion: Reduced BMD is highly prevalent among postmenopausal women in rural Bangladesh. Advancing age, longer menopausal duration and lower BMI are significant risk factors, highlighting the need for early screening and preventive strategies.