This paper examines the process through which Occupy activists came to constitute themselves as a collective actor and the role of social media in this process. The theoretical framework combines Melucci's (1996) theory of collective identity with insights from the field of organizational communication and particularly from the ‘CCO’ strand – short for ‘Communication is Constitutive of Organizing’. This allows us to conceptualize collective identity as an open-ended and dynamic process that is constructed in conversations and codified in texts. Based on interviews with Occupy activists in New York, London and other cities, I then discuss the communication processes through which the movement was drawing the boundaries with its environment, creating codes and foundational documents, as well as speaking in a collective voice. The findings show that social media tended to blur the boundaries between the inside and the outside of the movement in a way that suited its values of inclusiveness and direct participation. Social media users could also follow remotely the meetings of the general assembly where the foundational documents were ratified, but their voices were not included in the process. The presence of the movement on social media also led to conflicts and negotiations around Occupy's collective voice as constructed on these platforms. Thus, viewing the movement as a phenomenon emerging in communication allows us an insight into the efforts of Occupy activists to create a collective that was both inclusive of the 99% and a distinctive actor with its own identity.