The CuInSe2 nanoparticles for thin-film solar cells were successfully synthesized using a simple hydrothermal method with acetic acid as a mineralizer. The CuInSe2 nanoparticles with high purity were formed with acetic acid concentration of 5 M, reaction temperature of 200 °C, and reaction time of 12 h. The concentrations of acetic acid significantly affected the phase, morphology, and size of the prepared particles. The crystallinity and the particle size also increased as a function of the reaction temperature and the reaction time. Room-temperature Raman spectra shows that the single-phased of CuInSe2 nanoparticles were obtained via the hydrothermal synthesis with acetic acid. We proposed the one-step reaction mechanism for the CuInSe2 nanoparticles based on the X-ray diffraction and the scanning electron microscopy data.