Populism has become an increasingly important feature of politics in recent decades due to a rapid wave of populist parties spreading across the world from Europe to the United States, and South America. This motivates and necessitates renewed attention into understanding populism’s functions, origins and implications for international relations. In particular, as state identities reconstruct themselves in line with domestic political shifts (in this case leading to new populist identities), the importance of understanding the process of identity formation increases. To engage this issue, this research examines and utilises interrelated ideas and theories including identity formation, the role of social groups, group psychology and constructivist international relations theory. This thesis will examine whether identity politics influence domestic politics, and in extension empower populist movements. This will analyse how the state acts as an extension of the people that reside within it. It also provides scope to explain how domestic regimes and their changing identities inform international state behaviour by, firstly, looking at the effects of populism in recent years on the state of democracy globally and, second, through two case studies: the US (during the Trump administration), and Brazil (during the the Lula and Bolsonaro administrations). It will determine whether or not there were significant changes to US/Brazil foreign policy as a result of populism, and if so, how this affects international relations. In doing so, this research advances understanding of how domestic populism has had a significant influence on international behaviour. Additionally this research aims to continue and strengthen constructivist analysis by building on the existing academic literature through the understanding of psychological theories. Through it’s analysis it finds that populism has weakened democracy globally and that there are significant similarities between the foreign policy of right-wing populist parties in different countries, and differences between these parties and the foreign policy pursued by left-wing populists. It also identifies state behaviours, in terms of foreign policy, that align with principles of group identity behaviours and identifies areas where future research is warranted.