Herbarium and field assessments of the Western Australian Hibbertia diamesogenos (Steud.) J.R.Wheeler (Dilleniaceae) have shown that it comprises three distinct, sympatric species, two of which, H. parvula K.R.Thiele and H. xenandra K.R.Thiele, are described here as new. The segregation of H. parvula and H. xenandra from H. diamesogenos resolves the morphology of the latter, which is now substantially less variable than previously believed. It is likely that the three species are not closely related, with H. diamesogenos sens. str. almost certainly part of the Hibbertia polystachya Benth.–H. spicata F.Muell. species group, H. parvula possibly related to H. hypericoides, and H. xenandra possibly not closely related to any other known Western Australian species. Two further new species, H. micrantha K.R.Thiele & T.Hammer and H. humilis K.R.Thiele, are also described as new here, for convenience. They are likely not closely related to any of the other species dealt with and are both segregated from material determined as H. avonensis J.R.Wheeler, to which they are also unlikely to be closely related. All these species are characterised by very small flowers. A new description is also provided for H. diamesogenos, which is recircumscribed to exclude H. parvula and H. xenandra.