A two-phase closed thermosyphon (TPCT) is a device that effectively transfers heat through the phase change of the working fluid. TPCT comprises an evaporator, an adiabatic section, and a condenser. When heat is supplied to the evaporator, the working fluid is phase-changed to a gas phase, and the phase-changed vapor is converted to a liquid phase by releasing heat from the condenser. TPCT has various advantages of high resistance to thermal deformation, simple structure, and operation without additional power sources. Various studies, for instance, surface modification, nanofluid, and structure design, have been conducted to improve the thermal performance of TPCT. However, most studies focus only on improving the thermal performance of TPCT, and studies on geyser boiling are relatively insufficient. Geyser boiling refers to the phenomenon in which the liquid at the top of the bubble hits the cap of TPCT due to the instantaneous vaporization of the working fluid. Geyser boiling can occur periodically and damage TPCT, so it is considered to be avoided in its actual operation. The present study investigates the thermal performance of TPCT affected by geyser boiling and visualization carried out in the adiabatic section. This study provides an intuitive analysis of the effect of geyser boiling on TPCT. Therefore, it can be used as benchmark data for numerical analysis as practical information that has not been presented so far.