The interwar period in Europe was characterised by a multi-faceted movement in favour of European integration. After the slaughter of the First World War, many intellectuals, writers, industrialists and politicians brought the idea of European unity to the fore and engaged in various actions, from setting up organisations to lobbying governments, to promote the unification of Europe. Much research has been carried out on the leading figures of these pro-European activities but amongst the wealth of this period other actors have tended to be forgotten. Such is the case with the French writer Jules Romains, who not only coined “Europeanism”, the word that would define the whole movement in favour of Europe, but who also actively participated in promoting a united Europe. This article seeks to introduce and discuss Romains’ ideas on Europe. It will demonstrate that his vision was very coherent within the framework of his Unanimist philosophy but was undermined by serious ambiguities. It will also demonstrate that his ideas are of great interest for what they reveal about the interwar period in France and Europe, what they bring to the genealogy of the European project, as set up after the Second World War, and for the ambiguities at the core of his concept of Europe, which are still very much at the heart of many of today’s debates about the European Union.