The Celtiberian S is a new letter, created by the Celtiberians to write their own language with the Latin alphabet. The creation of a new sign presupposes that the Celtiberians who used the Latin alphabet to write their language felt that that system of writing was limited for their purposes. They ascertained that the Latin alphabet lacked a specific sign for representing one or several of the sounds in their language, presumably those represented as sigma in Palaeohispanic writing. They tried to resolve this absence with the creation of a new sign: they added a stroke below the S. A bronze fragment with an inscription written in Celtiberian language and Latin alphabet has recently been found in Novallas (province of Zaragoza). Its discovery has revealed the existence of this new letter. The new S, which appears not only in this new text, but also in a pair of inscriptions from Peñalba de Villastar: the inscription known as the Great Inscription (K.3.3), and one of the short ones (K.3.14). In this paper we communicate to the scientific community the identification of the aforementioned letter in Latin inscriptions from Celtiberia. We have detected the marked Ss in five Latin inscriptions: Buenafuente (CIL II 5790), Almadrones (CIL II 6294), Peralejo de los Escuderos, Lara de los Infantes and Salas de los Infantes (three certainly and two in all likelihood). They are always geminated and always appear in Celtiberian proper nouns. For the first time, the marked S is reported on inscriptions in Celtiberian language and Latin alphabet. The exact dating of this group of inscriptions cannot be specified with certainty, but the available evidence points to the first century BCE. In conclusion, a new grapheme was created to write Celtiberian in Latin alphabet: the marked S, which is only attested in three Celtiberian documents, but which survives in some Latin inscriptions from the Imperial period to denote vernacular proper nouns, and which we propose calling Celtiberian S.