Eco-socialism was developed in the 1970s following the rise of the global environmental movement in the 1960s (O’Connor 1998). It was integrated into the IR scholarly discipline by the 1980s as a critical unconventional theory (Saurin 1995). Eco-socialism is critical of capitalism and its intertwined effects on the environment and human welfare. It believes that global environmental degradation is caused by a global exploitative and unequal socioeconomic relationship amongst human beings. Therefore, eco-socialism seeks to transform this global pattern to one that is free from degradation, exploitation, and inequality. This article explores how eco-socialism defines the environment, the negative effects of capitalism on the global environment and human welfare, and how to solve the negative effects.