The notion of solidarity plays an important role in debates about the future of Europe. Yet, it can be used in a diverse or even contradictory manner as contemporary discussions about refugees in Europe or the implementation of social rights at the EU-level well illustrate. While the focus of public attention is often on political leaders, the goal of this article is to deepen our knowledge about the understanding of solidarity by its practitioners at the grassroots-level of solidarity work. Our study is based on 97 individual interviews with representatives of transnational solidarity organisations (TSOs) located in Germany, Poland, and Greece. We show how their understanding of solidarity varies across three basic themes: the rationale of action, the scope of beneficiaries, and relations with and among beneficiaries. Most strikingly, our research reveals that the core understanding of solidarity among the TSOs under review reflects a progressive, transnational type of solidarity that emphasises universalism, connection-building between targets of solidarity and establishing group identities based on collective action and not pre-defined features. At the same time, our study shows how TSOs adapt to complex country- and field-specific contextual conditions.