Earlier research has concluded that there is a strong symbolic relationship between Helsinki as a place and non-standard /s/ pronunciation. This phenomenon is likewise in continuous evidence in the Finnish media and social media. The notion of “Helsinki s” has become a folk linguistic fact although it lacks a clear linguistic correlate or even status as a linguistic fact. The only sibilant of the Finnish language is officially a voiceless alveolar, while the “Helsinki s” is most often discussed as “hissing”, “sharp” or “fronted”. However, according to recent research based on listening tasks, any /s/ may be designated and discussed as a “Helsinki s” if the speaker is regarded as a Helsinki speaker according to other criteria. The present article seeks to trace a development path to illustrate the layered semiotic processes that has led Finnish society to construct “Helsinki s”. In exploring the indexicalization process of this folk notion, the article focuses in particular on the origins and the circumstances under which /s/ pronunciation started to gain social recognition in Finnish society. The ideological link between the socially meaningful /s/ pronunciation and Helsinki is traced back to the early industrialization and urbanization era and (media) discourses of Helsinki in the late 19th century. The present-day associations and the 150-year path of “Helsinki s” are discussed.