This article investigates the role of semiotic tools as sign vehicles in mediating the socially shared meaning making process in the learning of mathematics pedagogy. Our particular interest is to investigate the evolving role of the social sign in learning within culturally organized activities. The participants in our study were a group of teachers participating in an in-service mathematics course whose pedagogy draws on the sociocultural perspective. A specific analysis method to unravel the nature of the semiotic tool as a sign vehicle was developed for the study. The results of the study suggest that socially shared meaning making around the semiotic sign vehicle consisted of seven phases, with a shared social sign as an end result indicating collaborative learning. On the whole, the semiotic sign vehicle investigated in this study supported the understanding of socially shared meaning making and hence teacher learning in the problem solving situation.