The author of De mundo offers his outline of geography in the second part of Chapter 3. In this contribution, we look at the details of the author’s account and analyse it in the context of the historical development of ancient geographic ideas. There is a number of indications that the text can be dated to later Hellenistic times: the author’s view on the insularity of the inhabited world, on the nature and role of the ocean and its connection with the Caspian/Hyrcanian Sea, on the borders between the continents and on the calculation of the dimensions of the inhabited world (oikoumenē). We argue that the author might have been influenced by the treatises on the ocean as a literary model for this section of De mundo. We know that authors such as Pytheas of Massalia, Posidonius of Apamea and Athenodorus of Tharsus wrote works under the title On the Ocean, which survive only in scarce fragments. The information we can extract from De mundo in this respect is, therefore, invaluable for a reconstruction of the content, circulation and reception of this sub-genre of geographic literature in the last decades of Hellenism.