This article analyzes the map of the Strait of Magellan and the surrounding area (the "Region Austral Magallanica") that Francisco de Seyxas y Lovera (c.1646-c.1705) added to the "Taboas geraes da toda a navegacao" (1630), a manuscript atlas by Portuguese cartographer Joao Teixeira Albernaz I. I review briefly the biography of Seyxas up to 1692, the year in which he remitted the atlas to Charles II in his Council of Indies, and the cartographic tradition related to the area in the years preceding the drawing of Seyxas's map. I then examine in particular three unique aspects of the map: the eponymous Islands of Seyxas, the Strait of La Roche, and two alleged English settlements. Lastly, I propose a reading that permits us to understand the map not only as an aberrant episode in the cartographic representation of the region, but also as a reflection of the political, autobiographical and imaginative discourses that characterize Seyxas's larger body of written work.