This paper presents a classification of different natural flow regimes found in Ebro basin, one of the largest in the Mediterranean region. Determination of flow regimes was based on multivariate analyses using long-term discharge series of unaltered flow data. Mean monthly discharges of the 30 ‘best’ flow series and a total of 52 flow series containing unaltered flow data were selected to represent baseline flow conditions for tributaries throughout the basin. Metrics representing magnitude, duration and frequency components of flow were used to identify hydrologic differences across the basin. A total of six natural flow regimes were identified in the Ebro Basin, using a Ward cluster method. The flow patterns identified and their spatial distribution largely corresponded with climatic zones previously reported for the Ebro Basin, with regime types ranging from pluvio-oceanic in the western part of the basin to Mediterranean in the eastern region. Geologic characteristics of the catchment and altitude of headwaters were also found to play an important role in defining flow regime type. A 19-hydrologic variable subset was used to explain main hydrologic differences among groups (such as magnitude and frequency of extreme flow conditions or magnitude and variance of average flow conditions). However, stepwise discriminant analysis was not able to identify consistent subsets of hydrologic variables that adequately identified the six natural flow regime types in this basin. Canonical discriminant analysis was useful to understand class separation and for the interpretation of results. Copyright © 2012 John Wiley & Sons, Ltd.