The Swan Coastal Plain of south-west Western Australia has been extensively cleared since European settlement, with the fertile Pinjarra Plain land system preferentially cleared for grazing; what remains of the native vegetation is of high conservation value despite being highly fragmented and often degraded. The following six taxa of Synaphea R.Br. (Proteaceae), with conservation status corresponding to IUCN Red List categories Threatened and Data Deficient, are restricted to vegetation remnants in this region: S. stenoloba A.S.George, S. odocoileops A.S.George, S. sp. Pinjarra (R. Davis 6578), S. sp. Fairbridge Farm (D. Papenfus 696), S. sp. Pinjarra Plain (A.S. George 17182) and S. sp. Serpentine (G.R. Brand 103). Taxonomic boundaries among these morphologically similar taxa of Synaphea were investigated using morphometric analyses based on characters of leaves, inflorescences, flowers and fruits. Results allow the circumscription of the four informally named taxa from this complex, which will enable their formal description in a subsequent publication. They also provide support for several morphologically closely related taxa, including the provisional distinction of S. sp. Udumung (A.S. George 17058) from close ally S. decorticans Lindl. Two described species (S. odocoileops and S. stenoloba) and several anomalous specimens could not be adequately resolved in these analyses, and further investigation of their boundaries using molecular markers is required.