Dengue virus (DENV) infection is increasing with rapid urbanization in India. Treatment of DENV infection is mainly supportive with no specific antiviral therapy. Although most patients show mild illness, some have a severe disease course such as dengue hemorrhagic syndrome, dengue shock, multi-organ failure, and death. The cause for severity is not fully understood. Currently, there are no methods available to predict the course of the illness. Hence, it is crucial to develop an early biomarker to predict the course of dengue illness which can aid in vigorous monitoring and early intervention. Here, we tried to establish a correlation between serum ferritin and severity of dengue illness. We measured ferritin levels in 100 dengue-positive cases on day 1 (D1) (febrile phase) and day 4 (D4) (defervescence or convalescent) of admissions to compare the levels with the severity of the disease. On D1, the serum ferritin level was a "good" predictor of severe dengue, with an area under the curve (AUC) of 0.863 with standard error (SE) = 0.043 and a 95% CI from 0.778 to 0.947 (P < 0.05). On D4, serum ferritin was an "excellent" predictor of severe dengue, with an AUC of 0.947 with SE = 0.021 and a 95% CI from 0.907 to 0.988 (P < 0.05). Serum ferritin is an inexpensive and easily accessible biomarker that can assist in monitoring and prognosticating the dengue-positive patients. This biomarker also directs us to explore the underlying pathogenetic mechanism in severe dengue, which can lay a foundation for future targeted therapeutic options to combat severe illness.