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. Two further new species, H. micrantha K.R.Thiele & T.Hammer and H. humilis K.R.Thiele, are also described as new here, both segregated from material previously determined as H. avonensis J.R.Wheeler. 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.