In this chapter, we discuss the notion of media as sign mediation and transmedia dynamics as a pragmatic offshoot of media semiosis, according to Charles Sanders Peirce’s (1839–1914) pragmatic approach. In this sense, transmedia dynamics refers to integrated media experiences that unfold across various platforms, attracting audience engagement in an immersive and creative way. Based on the Peircean semiosis model, which is also a communication model, we argue that transmedia dynamics operates as a kind of sign mediation, or semiosis, in which the storytelling changes continually for the integrated action of media producers and consumers. The productive incompleteness of the interpretant is taken as a conceptual parameter to understand the way in which the collective actions interfere in the transmedia dynamics. We demonstrate this argument by analyzing the collective action in online and off-line connections mediated by hashtags in the social mobilizations’ context in Brazil.