Today, Northeastern India receives some of the highest annual rainfall totals globally. The major portion of annual precipitation in this region falls during the Indian Summer Monsoon season (June–September); however, this region also receives a significant amount of rainfall during the pre-monsoon season (March–May). Here, we quantitatively reconstruct the climate of the Upper Siwalik subgroup, eastern Himalayan region, based on fossil assemblages using the Coexistence Approach (CA). The age of the fossil assemblage is considered to be late Pliocene–early Pleistocene. Data reconstructed for the present-day and past pre-monsoon rainfall in Northeastern India indicate an increasing trend since the late Miocene–early Pliocene. During the late Pliocene–early Pleistocene (Upper Siwalik), the temperature seasonality between warm (27–28.1 °C) and cold months (22–23.6 °C) was less pronounced compared with present-day warm (27–27.7 °C) and cold (14.8–15.4 °C) months conditions at the fossil locality. The reconstructed rainfall data indicate a monsoonal type of climate having a strong seasonality in wet and dry seasons during the deposition of the Upper Siwalik sediments. Moreover, composition of the fossil floras and reconstructed palaeoclimate suggest a vegetation shift from dominantly wet evergreen to semi-evergreen at the fossil locality, coincident with an increase in length of the dry season. The comparison of reconstructed CA data and climate modelling data of a Gelasian time slice with that of previously reconstructed climate data by using Climate Leaf Analysis Multivariate Programme (CLAMP) analysis of the late Pliocene–early Pleistocene (Upper Siwalik) fossils of the same locality provides nearly the identical result. Furthermore, all the reconstructed data indicate a monsoonal type of climate during the deposition of the Upper Siwalik sediments.