Persuasive technologies for health-related behaviour change give rise to ethical concerns. As of yet, no study has explicitly attended to ethical concerns arising with the design and use of these technologies for vulnerable people. This is striking because these technologies are designed to help people change their attitudes or behaviours, which is particularly valuable for vulnerable people. Vulnerability is a complex concept that is both an ontological condition of our humanity and highly context-specific. Using the Mackenzie, Rogers and Dodds’ taxonomy of vulnerability, this paper identifies (a) the wrongs or harms to which a person is vulnerable, (b) the source of this vulnerability, and (c) the safeguards needed in response. Two ethical concerns with the design of persuasive technology for vulnerable people are discussed: the concerns of taking into account users' interests and their autonomy.