The aim of this paper is to present the formation processes and functions of ethnic identity in the Greek diaspora. The paper taking as its starting point Cavafy’s poem “Poseidonians”, which condenses concepts and questions associated with the issue of identity, is structured as follows: First, readers are introduced to the concept of ethnic identity (Sects. 1 and 2) and to key concepts deployed during the critical analysis. Subsequently, it presents both the composition and organization of the Greek diaspora (Sect. 3) and underlines the role of the Greek language as a link between the Center and the Diaspora (Sects. 4 and 5). These three sections demonstrate the context in which ethnic identity is constructed and operates. The following sections (Sects. 6, 7, 8, and 9) form the core of our critical analysis: they include the presentation and investigation of various manifestations of ethnic identity by exploring the relationship between ethnic identity, language, and education. At the end (Sect. 10), key implications are synthesized, and conclusions are drawn on the construction, manifestations and multiple functions of ethnic identity in the Greek diaspora, highlighting the significant impact of Greek heritage language as a component of ethnic identity.