Jacques Lacan devoted one of the sessions of his important Seminar XVII, The Other Side of Psychoanalysis, to Ludwig Wittgenstein’s classic work Tractatus Logico-Philosophicus. Despite Lacan’s and Wittgenstein’s huge influence across academic disciplines (and the analytic-continental divide in philosophy), there has been no scholarly attempt to examine Lacan’s claims concerning Wittgenstein in this seminar. This paper sets out to redress this gap in the literature. Specifically, we look at the context of the engagement; Lacan’s reconstruction of Wittgenstein’s arguments; their differing conceptions of truth, knowledge and the subject; and the issue of Wittgenstein’s seemingly “psychotic” position.