This article establishes a new basis for examining the participation, mobilisation and impact of investors at a time when market-based activism for social change is rising in prominence. Existing terminology describing the expression of political values through investment decisions lacks conceptual clarity. Political participation by shareholders and other investors is variously described as shareholder activism or socially responsible investment, and currently conceptualised under the banner of political consumerism. However, this term fails to capture the unique political role and diverse actions of investors. We put forward ‘political investorism’ as a cohering term for investment-based political participation to remedy existing conceptual confusion, to distinguish between investors and consumers as political actors and to set an agenda for the future study of market-based activism. This article defines and develops the concept of political investorism, drawing upon illustrative cases from Australia to identify hallmarks, actors and tactics of this form of political participation.