Background <p>Anemia is a critical public health issue that disproportionately affects adolescent girls due to their elevated physiological demands during rapid growth and the onset of menarche. This cross-sectional study investigated the prevalence of anemia and its association with body mass index (BMI) among adolescent schoolgirls in Zarqa, Jordan.</p> Methods <p>A total of 420 participants aged 12–16 years were recruited through random sampling from six government schools between August 2024 and January 2025. Data collection included anthropometric measurements, complete blood count, dietary intake assessment, and sociodemographic characteristics.</p> Results <p>The prevalence of anemia was 35%, with most cases classified as mild. Distribution of anemia across BMI categories revealed that 60.7% of anemic girls had a normal BMI, 22.3% were overweight, 14.2% obese, and only 1.4% severely thin. No significant association was found between BMI categories and anemia (<i>p</i> = 0.796), and correlation analysis confirmed a weak, non-significant relationship between BMI-for-age Z-score and hemoglobin levels (<i>r</i> ≈ 0.06, <i>p</i> = 0.55). Comparisons with regional and international studies highlight important contextual differences: in countries with higher undernutrition rates, low BMI has been a significant predictor of anemia, whereas in Jordan, dietary inadequacy, menstrual losses, and lifestyle factors appear to overshadow BMI as determinants.</p> Conclusions <p>The study underscores that the anemia–BMI relationship is not universal but rather shaped by the broader nutritional transition and socioeconomic context. These findings suggest that public health strategies in Jordan should prioritize improving diet quality, enhancing iron intake, and addressing adolescent health awareness over relying solely on anthropometric markers as predictors of anemia.</p>