Even since the seventeenth century, historiography has attempted in vain to uncover the identity of the author of the Vindiciae contra tyrannos, one of the main monarchomachist treatises of the period. The two most common hypotheses identify either Hubert Languet or Philippe Duplessis-Mornay as its author, with the most recent consensus of French historiography tending to favour Mornay. However, our study of the two versions of the Arcadia (i.e., Old Arcadia, ca. 1580 ; and New Arcadia, 1590) written by Philip Sidney, Languet’s closest friend, reveals not only that Sidney expresses political ideas very similar to those of the Vindiciae, but also that he explicitly attributes them to Languet. Such a testimony is deserving of consideration by historians. Although it does not definitely prove that Languet authored the Vindiciae, it does reveal that he shared its philosophy. Moreover, while French historiography has all too often dismissed the Languet hypothesis, Sidney’s Arcadia show that it should not be shrugged aside and may even have some merit.