This study provides preliminary evidence of morphometric variation within and among species of the Antirrhinum section Streptosepalum (Plantaginaceae) that allows further development of hypotheses concerning species boundaries. Multivariate statistical analyses of the measurements of ten morphological characters suggest that the Antirrhinum occurring on the serpentine outcrops in the province of Trás-os-Montes, NE Portugal, is a species different from A. braun-blanquetii and A. meonanthum. Its ecological preferences are also different. Antirrhinum rothmaleri (Pinto da Silva) Amich, Bernardos & García-Barriuso comb. & stat. nov. is described and illustrated. Notes on its distribution, ecology, karyology, and taxonomic relationships are presented. The diagnostic differences among A. rothmaleri, A. braun-blanquetii and A. meonanthum are provided.