While solidarity matters to any form of human grouping, it is rarely discussed in international relations. Currently, due to the ever-developing international relations, studying this topic will contribute to the research and application of solidarity in international relations. This article explores the concept of solidarity, perspectives on solidarity in international relations theories worldwide, and compares theoretical concepts and viewpoints with reality to examine and evaluate the presence of solidarity in international relations. Based on this, it offers some insights into the non-material nature of solidarity, its significance for nations and international relations, and the difficulties in achieving international solidarity. The article also offers further suggestions for studying solidarity in international relations.