The identity of Dinophysis sacculus Stein has proved to be problematic in recent years, during which this species has been associated with diarrhoetic shellfish poisoning (DSP) events along European coasts. To complement the scarce and at times contradictory existing information, we provide details of morphological variation in D. sacculus throughout its geographical range, which includes the Mediterranean Sea and European Atlantic coasts. Our data clarify its relationship with D. pavillardii Schröder. This species differs from D. sacculus only by the presence of a concavity along the dorsal margin, but transitional morphotypes are frequent in almost all the samples examined. We therefore propose merging the two species and classifying morphotypes with a dorsal concavity as D. sacculus f. reniformis (Pavillard) comb. nov. Despite the high variability of Dinophysis sacculus, it was generally possible to separate it from D. acuminata Claparède & Lachmann. The only character distinguishing the two species is the shape of the large hypothecal plates, which in D. sacculus are elongate, almost rectangular or sac-like, whereas in D. acuminata they are shorter, more convex dorsally and often more slender towards the antapex. Thecal areolation and sulcal list ornamentation are more pronounced in D. acuminata, although these characters are somewhat variable. Other thecal details, such as the shape of sulcal platelets and the structure of the apical pore area, were not useful for circumscription of the two taxa. In a few samples from the Galician Rias and from the Spanish Mediterranean coast, we observed morphotypes intermediate between D. sacculus and D. acuminata, which indicates that the two species can have slightly overlapping ranges of variability.