Understanding the photoexcitation processes in semiconductors is critical for the design of advanced photocatalytic materials. Nevertheless, traditional viewpoints focus on photogenerated free charge carriers, which are somehow invalid once the many-body effects are taken into account, especially for polymeric photocatalysts. Here we systematically investigate the photoexcitation processes involved in the polymer matrix of graphitic carbon nitride (g-C3N4) by combining photoluminescence spectroscopy and ultrafast transient absorption spectroscopy, validating the strong excitonic effects in the well-known photocatalyst for the first time. The identification of the robust triplet-triplet annihilation process, in which two triplet excitons collide to produce a singlet exciton, highlights an important nonradiative depopulation pathway of excited species and thereby offers potential strategies to regulate the photocatalytic activities of polymeric g-C3N4. The work establishes a new understanding of the photocatalytic mechanism in the polymeric g-C3N4 matrix, and thus paves the way for designing effective polymeric photocatalysts through excitonic engineering.