Floods are a major threat in many valleys of the Indian Himalayan Region (IHR). Yet, the lack of reliable data on past events renders the implementation of appropriate adaptation policies a difficult task, and therefore also hampers the mitigation of future disasters. In an attempt to overcome these shortcomings, we combine reconstructed paleoflood events based on tree-ring analyses with existing systematic records, so as to derive a regional flood frequency. Analysis was realized with tree-ring records and through the dating of growth disturbances in riparian trees of major rivers in Kullu district (Himachal Pradesh, Indian Himalayas). To this end, we combined field-based observations, tree-ring analyses, hydraulic modelling and statistical approaches. Results suggest that the occurrence of floods in Kullu district is recurrent, with a marked seasonality and a cyclic natural variability in flood frequency at multi-decadal scales, as well as distinct spatial representativeness. The inclusion of peak discharge data of past, previously ungauged, flood events derived from tree-ring records has a significant and positive impact on the flood frequency assessment. Flood hazards and associated risks have been clearly underestimated in the region and based on the systematic records alone. We also demonstrate that a regional flood frequency approach is suitable to optimize the information gathered from tree rings and that flood frequency can thus be analyzed for larger regions. The approach used in this paper can be implemented in the other, poorly gauged region and thus contribute to climate change adaptation policies in undocumented environments such as the Indian Himalayan Region.