In this article, we argue that rational persuasion can be a _pro tanto_ wrong and that online platforms possess features that are especially conducive to this wrong. We begin by setting out an account of rational persuasion. This consists of four jointly sufficient conditions for rational persuasion and is intended to capture the core, uncontroversial cases of such persuasion. We then discuss a series of wrong-making features which are present in methods of influence commonly thought of as _pro tanto_ wrong, (...) such as manipulation and paternalism. It is next shown that these wrong-making features are also present in a range of cases that are, by the jointly sufficient conditions already established, rational persuasion, and so some forms of rational persuasion are _pro tanto_ wrong, including in some ways that have not previously been remarked upon. Finally, we demonstrate that online settings possess a number of features that are especially conducive to wrongful rational persuasion. (shrink)