This is a research in the field of Internet and Politics in the subfield of the public sphere and transnational activism. The aim of this work is to identify the constitution of transnational publics, which are expressed through activism, using digital media. Based on a literature review, two emblematic cases of transnational activism will be analyzed: the coordinated protests of 15 February 2003, held simultaneously in many countries against the imminent Iraq War, and the 2011 protests in Egypt that took place until the resignation of the president Hosni Mubarak, and were located in the context of the Arab Spring, which included similar events in other Arab countries and strong use of social media. Both cases will be studied in light of the following questions: 1) which are the characteristics of these publics that may be formed around certain issues?; 2) how was social media used?; 3) what makes these publics transnational? By doing so, we hope to contribute to the research on transnational activism and public sphere.