The current study reveals that in Unamuno’s literature, jealousy is not portrayed as a mere human emotion, but as a very powerful force which plays a pivotal role in his thinking. He has dealt with “jealousy” in many of his works, among which Tragic Sense of Life and Abel Sanchez: The History of a Passion stand out due to their deeper approach and detailed analysis of this subject. The latter work, in particular, reveals that the jealousy of Biblical Cain is still present in modern people’s minds and in their collective consciousness. In this paper, we first will have a look at the existential dimension of jealousy in Unamuno’s Abel Sanchez, which will leads us to discover that this human emotion is interpreted by him as a desire to become someone else, which reveals itself as a terrible malady. We will then consider the aspects of Unamuno’s withdrawal into his inner realities in a way that may allow overcome this consuming emotion. In the light of the results of this paper, we can conclude that jealousy in Unamuno’s literature is closely connected with his concept of intrahistory and his thinking concerning the relationship between Spain and the rest of Europe, as well as his understanding of man from his own existentialist perspective.