This research describes a new persuasion knowledge measure for older consumers which can be applied for both diagnostic and preventive purposes. Persuasion knowledge refers to information, attitudes, beliefs and schemas guiding our behaviour in situations where persuasion is involved. Many older consumers belong to a specific, vulnerable group, who often fail to maintain their interest in persuasive situations, so the proposed measure could serve a better understanding of their behaviour. First, a new, situation‐based achievement measure, entitled ‘Persuasion Knowledge Achievement Measure’ (PKAM), was developed in university samples (N = 530) in several stages. Afterwards, the developed measure was adapted to older individuals, resulting in the Persuasion Knowledge Achievement Measure for Older People (PKAM‐OP). Following a pilot study with personally‐assisted data collection (N = 50), the examinees were reached through an elderly care network in a two‐stage procedure (N = 183, N = 116). The psychometric analyses of the data show a high level of reliability and validity for both tests. The results of the main research studies indicate that there are two subscales of the measure: ‘Agreeableness’ and ‘Persuasion Knowledge’. Based on the empirical data, the subscales provide reliable measures of features which guide behaviour in persuasive situations. In sum, PKAM‐OP can fulfil theoretical and practical needs in research and prevention interventions focusing on older consumers.