Using the writings of Jean-Paul Sartre, Emil Cioran, and Ferdinand Pessoa as examples, the author proves that boredom is one of the most important signs of modernism. It duplicates a feature ascribed to modernist movements or groups of movements, and to the era itself – indeterminacy. The author presents a kind of “phenomenology of boredom” that can be derived from the works of these three writers and points out its “bordering” status: It is an experience that takes place at the meeting point of physiology and will, causing a feeling of the excess of existence, the senselessness of being, or the absurdity of the world. Boredom acts on the subject destructively, fragmenting its identity, undermining its substantiality, and bringing to light its internal otherness. As a phenomenon specific to modernism, it can be compared with anomy and nihilism, with which it produces a “modern triad” – a set of unavoidable modern experiences. The last part of the article proves that boredom is also a kind of deconstruction of the subject: a visceral “foundation” of existence that both initiates the construction of identity and prevents the successful completion of this process.