In this paper, different HgO nanoparticles were synthesized using the hydrothermal method in the absence and presence of tartaric acid, citric acid monohydrate, and succinic acid as low-cost capping agents. FT-IR, XRD, UV–Vis spectroscopy, FE-SEM, and HR-TEM were utilized for the characterization of the synthesized HgO samples. The results proved that the utilized organic acids have a significant effect on producing HgO nanoparticles with different crystallite sizes and energy gaps. The HgO sample, which was synthesized using citric acid monohydrate, was tested for the photocatalytic degradation of crystal violet dye. Complete photocatalytic degradation was obtained using HgO after 180, 120, 80, and 60 min in the presence of UV, sunlight, (sunlight and KBrO3), (sunlight and ( NH4)2S2O8), and (sunlight and H2O2), respectively. Also, the pseudo-first-order rate constant of the degradation processes was increased in the following order; (HgO + UV) < (HgO + sunlight) < (HgO + sunlight + KBrO3) < (HgO + sunlight + (NH4)2S2O8) < (HgO + sunlight + H2O2). Besides, the crystal structure and catalytic activity of HgO remained unchanged even after it was reused three times. In addition, atomic absorption spectrophotometer measurements of the liquid phases of all catalytic experiments prove the absence of mercury. Hence, HgO is a stable photocatalyst and can be used repeatedly without sacrificing its degradation ability towards the crystal violet dye.