Marine deposits in the Neogene Siwalik succession have been documented from the Tista valley, Bhutan and Kameng river sections of the Eastern Himalaya. This study was designed to investigate possible marine influence further east. During this investigation, it was revealed that the current classification and description of the stratigraphic units were inadequate to describe the variations present in this succession fully. In place of the existing three-fold classification, a four-fold lithostratigraphic classification of the Siwalik Group in the Arunachal Himalaya has been proposed by adding a new Siji Formation. This newly proposed formation is completely different from the underlying sand-dominated Subansiri or overlying conglomerate-dominated Kimin formations. Nearly 1000 m thick succession of the Siji Formation is characterised by of alternating tabular sheet like beds of grey mudstone, siltstone and sandstone with uncommon thin conglomerate beds. On the basis of the four-fold classification, the Siwalik rocks of the Arunachal Himalaya have been mapped over a strike length of ∼230 km, demonstrating the lateral continuity of the individual formations and the large-scale deformational features affecting the rocks. Three lithologs prepared show the distinctive lithological characters and overall paleocurrent pattern of these formations. The lithological, sedimentological and ichnological features of the Dafla and Siji formations provide evidence for marine influence in the study area. The proposed new lithostratigraphic subdivisions are expected to facilitate regional correlation and evidence of marine sedimentation which will throw new light on the basin analysis of the Siwalik Group in the Eastern Himalayan.