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.