Friedrich Melchior Grimm’s correspondence with the king of Poland, Stanislas Auguste Poniatowski, reveals the inextricable connections between the Correspondance littéraire and the private letters often sent with it, and constitutes a unique source of insight into the persona of the prince to whom this periodical was addressed. The study of this epistolary relationship reveals the complexity of the idealized princely ethos shaped by the changing figures of subscribers and constructed within a dynamic dialectic with that of the writer. At once representative of the way Grimm forged links with his readers, and rich with Poniatowski’s reactions, this set of private letters also allows us to question the effectiveness of the representations of friendship on display in the periodical, and encourages an appreciation of how Grimm’s balanced manner and rhetorical skills allowed him, despite criticisms and refusals likely to displease the Polish king, to find in him not only an engaged and receptive reader, but a “royal friend.”