Multimetric indices based on fish assemblages assessing the ecological status of rivers have been widely developed. Nevertheless, the heterogeneity of environmental conditions, biological assemblages, and human pressures require the development of suitable indices for different regions. In the Iberian Peninsula, where there is a lack of an accurate tool for monitoring systems under temperate oceanic conditions, indices are developed under a Mediterranean climate. Therefore, our goal was to develop and validate a multimetric index based on fish assemblages to evaluate the ecological status of streams and rivers from the Northern Iberian Peninsula under a temperate oceanic climate. For this, 147 streams (41 reference and 96 disturbed streams) were monitored from 2010 to 2014. At each site, we calculated the stress level based on human alterations. For the index development, the rivers were grouped into two large classes: salmonids and cyprinids. Ninety-one metrics were tested for their capability to discern stress level. The index was calculated as the mean of five metrics for salmonid rivers: the percentage of density of benthic species, the percentage of richness of species intolerant to contamination, the percentage of richness of species intolerant to low oxygen concentrations, the percentage of insectivore density, and the percentage of Salmo trutta density. As the mean of two metrics for cyprinid rivers, we considered the percentage of density of water column species and the percentage of density of species intolerant to low oxygen levels. In salmonid rivers, the longitudinal connectivity was evaluated by measuring the presence of Anguila anguila. The index was split into five ecological status classes, ranging from 0 (bad ecological status) to 1 (high ecological status). This index was compared with four other biological indices and its performance was validated for 793 study cases, thus demonstrating its suitability for employment in biomonitoring works to assess the ecological status of stream and river ecosystems from the Northern Iberian Peninsula under temperate oceanic conditions.