Hydrostatic pressure spectroscopy is carried out using a custom-built optical high-pressure apparatus that enables diverse spectroscopic measurements under precisely controlled isotropic pressure conditions. The system consists of two primary components: a quartz-based inner cell that contains the sample solution and a stainless-steel outer pressure cell that transmits hydraulic pressure. The inner cell, composed of a quartz cuvette, a Teflon tube, and a metallic cap, withstands pressures up to approximately 500 MPa and is hermetically sealed before being placed inside the water-filled outer cell. Pressure is applied through a hand-operated pumping unit, which delivers uniform hydrostatic pressure to the sample by compressing the surrounding water. The outer pressure cell is equipped with sapphire or quartz windows for UV-vis absorption, fluorescence, excitation, and time-correlated single-photon counting measurements, while diamond or YAG windows are employed for circular dichroism (CD) spectroscopy to avoid optical artifacts such as linear dichroism and birefringence. The pressure cell can be readily mounted on a range of spectroscopic instruments, enabling stable and reproducible acquisition of optical data. This design provides a versatile platform for systematically probing pressure-dependent photophysical and photochemical processes in molecular, supramolecular, polymeric, and biological systems under fully isotropic hydrostatic pressure conditions.