A new species of Macrosiphum Passerini is described from western North America based on the apterous and alate viviparous females. Macrosiphum glawatz n. sp. lives on Potentilla gracilis (Rosaceae), apparently without host alternation, being commonly attended by ants in the genus Formica L. Details are provided regarding its similarity to other Macrosiphum, especially Macrosiphum mentzeliae Wilson, as which it was initially mis-identified. New information about biology and taxonomy of M. mentzeliae is provided for comparison purposes. New information is also provided about two other Macrosiphum recorded from Potentilla: Macrosiphum potentillae (Oestlund) and Macrosiphum potentillicaulis Miller. Comparative taxonomic details about M. potentillae are provided based on Oestlund's original material and the lectotype. We report the location of the type slide of M. potentillicaulis and declare it a junior synonym of Macrosiphum valerianae (Clarke), new synonymy.