A book discovered recently in the National Library of Wales includes Gabriel Harvey's annotations of Vindiciae contra Tyrannos. The title page of this volume calls Vindiciae a rebuttal of Machiavelli's Prince, with which it is bound. It is in fact a critique of the institution of monarchy and was the subject of fierce debate in Harvey's lifetime (1552/3–1631), since it presents tyrannicide as legitimate. This essay considers the consequences of these findings for Harvey's reputation, since the appreciative and nonjudgmental way he comments on the text in his marginalia contrasts sharply with the condemnation of it in his printed works. The existence of this book may also shed new light on two lost works of Harvey's, Anticosmopolita and Tyrannomastix. This article argues that, far from being an epic poem as commentators have so often said, Anticosmopolita was Harvey's response to another antimonarchical text, and Tyrannomastix was his rebuttal of Vindiciae.