For over a century, studies on Marcion have cited a quotation attributed to him in a fragmentary Syriac manuscript: London, British Library, Add. 17215 (fols. 30–3). An English translation of the relevant passage appeared in 1893, but no subsequent study has returned to the Syriac text itself. While this text has hitherto been cited as an anonymous Syriac source, this article identifies it as a letter by Jacob of Serugh (d. 520/1) and offers preliminary remarks on the implications of this identification for future research on Marcion's Gospel and his thought.