Over the last three centuries various attempts have been made to identify a sculptor represented in a painting conserved in the Galleria Palatina in Florence. Giambologna and Pietro Francavilla figure among the most likely candidates. The present article proposes identifying the portrait as that of the painter, architect and wax sculptor Costantino de' Servi, an eclectic artist who worked in various capacities for the Medici family and for the major courts of Europe. Evidence suggests that the execution of the painting is linked to the marriage of Francesco I de' Medici and Christine of Lorraine. The article also investigates Costantino de' Servi's activity as a wax sculptor, proposing the attribution of two works from the Medici collections to him. Lastly, another 'Portrait of a sculptor', the work of Justus Sustermans, is identified as the 'Portrait of Costantino de' Servi' in later life.