A new species of the plantbug genus Dicyphus (Insecta: Heteroptera: Miridae), D. umbertae Sanchez & Cassis sp. n., from Portugal is described and the closely related species, Dicyphus cerastii Wagner is redescribed. Several body measurements were taken. Male and female genitalia of both species are illustrated. D. umbertae was found in several localities in Central and South of Portugal on tomato (Lycopersicon esculentum Mill.) and Hyosciamus albus L. D. cerastii was found in southeast Spain on squash (Cucurbita maxima Duchesne), L. esculentum, Ononis natrix L., Withania frutescens (L.) Pauquy and H. albus. The external morphology of the two species is very alike and measurements overlap. In the two species the aedeagus has a similar pair of prominent slightly arcuate sclerites, but the shape and setae on the left paramere are different. Phylogenetic analyses were performed using 381 bp fragments of Cytochrome b. Other dicyphine species were used as outgroups in the analyses: Dicyphus tamaninii Wagner, D. rubicundus Blöte, D. escalerae Lindberg, Nesidiocoris tenuis (Reuter), Cyrtopeltis geniculata Fieber and Macrolophus sp. Several evolutionary models were explored under a maximum likelihood framework. The molecular analyses provided strong support for the species identity of the two sister species D. umbertae and D. cerastii.