The Himalayan cryosphere system is a major source of freshwater supply for the Indian subcontinent. The rivers originating from the Himalaya are strongly influenced by the behaviour of glaciers depending upon the regional climatic variables. Therefore, understanding the mass budget of glaciers is important. In the present study, we discuss the hydrological mass balance of the largest Siachen glacier in the Karakoram range. The computations of runoff and other parameters on such a large glacier are challenging due to data scarcity, difficult accessibility and complex topography of the region. The snowmelt runoff model (SRM) coupled with in-situ and remotely sensed information was used to simulate the glacier runoff for the past three decades (1986–2018). The accumulation in the form of snowfall and ablation in the form of snowmelt and evaporation were used as inputs for the estimation of hydrological mass balance. The SRM has well estimated the discharge of Siachen glacier with the coefficient of determination (R$^2$) 0.92 and average volume difference (D$_v$) 4.4% for the validation period. The study revealed that the Siachen glacier experienced few marginally mass gain episodes before 2000 but after that, a rapid and continuous mass loss has been observed. The average mass balance during the past three decades was found to be -0.11 ± 0.24 m w.e./year. This suggests that glaciers of East Karakoram have started losing mass; still, it is not as significant as generally observed in other parts of the Hindu Kush Himalaya.