The current research seeks to develop an analysis of Ukraine's Euromaidan social movement in psychological terms. Building on the classic understanding of social competition strategies, we argue that Euromaidan protests can be conceived as an attempt of pro-European Union (EU) Ukrainians to realign the boundaries of the Ukrainian national identity by defeating the alternative pro-Russia integration project championed by the government. In particular, building on the encapsulated model of social identity in collective action, we suggest that Euromaidan is an emergent opinion-based group identity, formed in response to injustice through two self-categorical processes - group-level self-investment into the shared entity (i.e., Ukrainian national category) and disidentification from the alternative Russia-led Customs Union. Using a sample of 3,096 participants surveyed during the protests, we tested our hypotheses with structural equation modelling, where the model accounting for the direct and indirect paths of the self-categorical processes was expected to explain collective action intentions to a great extent than models applying the social identity and encapsulation models of collective action. We found evidence consistent with the proposal that Euromaidan was a pro-EU opinion-based group, formed in response to the government's decision to suspend the EU-Ukraine agreement and around individuals' general perception of unfair government authorities.