Mental health peer support is a growing modality of care in which people with lived experience of mental health conditions offer psychological support and education to one another. This form of care has been underexplored in medical ethics, despite the social and ethical complexities that distinguish it from other forms of clinical practice. In particular, peer support invites scholars to ask new questions about boundaries in healthcare: that is, the parameters set within relationships between people who receive care and people who provide it. As this article will demonstrate, boundaries in mental health peer support are remarkably fluid, indicating that existing language around crossings and violations do not fully account for the notion of “boundary flexibilities,” or setting boundaries that change depending on context. This article will explore the ethics of boundaries in mental health peer support through critical analysis of existing literature and through qualitative interview data with “peers,” or people trained to draw on their lived experience of mental conditions to aid others. We argue that more attention should be paid to boundary ethics in emerging forms of care, especially as they interface with clinical services.