Fred Halliday's life and work were intimately associated with the theory and practice of internationalism. In his later writings, the notion of ‘complex solidarity’ emerges as a key component of Halliday's worldview. This article explores the conceptual interconnections between different historical expressions of internationalism, cosmopolitanism and solidarity. It considers the intricate relationship between these categories and their place in our understanding of international affairs, emphasizing the divergence between liberal and revolutionary conceptions of internationalism and cosmopolitanism. The article discusses diverse understandings of ‘solidarity’ in International Relations, arguing that beyond the cosmopolitan and communitarian approaches, there exist other ‘Grotian’ and ‘republican’ ideas of solidarity. Halliday drew on these to present his own defence of universal human rights and solidarity, arguably developing a distinctive brand of republican internationalism. The latter part of the article gives content to ‘complex solidarity’ by suggesting it is built on three inter-related components: a methodological internationalism, an egalitarian reciprocity and a critique of global capitalism. Overall, these guiding features of complex solidarity deliver a unique rendition of internationalism which reflect Halliday's eclectic combination of radical liberalism with a residual historical materialism.