Electronic correlation is believed to play an important role in exotic phenomena such as insulator-metal transition, colossal magnetoresistance, and higherature superconductivity in correlated electron systems. Recently, it has been shown that electronic correlation may also be responsible for the formation of unconventional plasmons. Herewith, using a combination of angle-dependent spectroscopic ellipsometry, angle resolved photoemission spectroscopy, and Hall measurements, all as a function of temperature supported by first-principles calculations, the existence of low-loss high-energy correlated plasmons accompanied by spectral weight transfer, a fingerprint of electronic correlation, in topological insulator (Bi0.8Sb0.2)2Se3 is revealed. Upon cooling, the density of free charge carriers in the surface states decreases whereas that in the bulk states increases, and the recently reported correlated plasmons are key to explaining this phenomenon. Our result shows the importance of electronic correlation in determining correlated plasmons and opens an alternative path in engineering plasmonic-based topologically insulating devices.