Carex flava is a species complex with frequent hybridization. Within its Old World distribution, the group reaches the Iberian Peninsula, North Africa and Macaronesia, where the taxonomy of the group is particularly controversial. We performed a biometric study using quantitative characters and statistical multivariate and univariate techniques to test taxonomic relationships within previously defined genetic lineages, and to study the clinal morphological variation between those lineages. Our analytical approach allowed us to clearly discriminate between five well-defined morphogroups (C. demissa s. s., C. flava, C. lepidocarpa, C. viridula and a fifth group composed of C. nevadensis and populations from the Atlas region) and to establish morphological relationships for problematic morphotypes. The Pyrenean-Cantabrian populations predominantly displayed intermediate morphologies between C. demissa and C. lepidocarpa. Conversely, the Atlantic-Iberian morphotype, considered a hybrid between C. demissa and C. viridula, displayed a morphology that was often similar to C. lepidocarpa. The analytical approach adopted here may be relevant for the study of morphological intermediates in other plant species complexes with similar taxonomic problems.