Biocompatible cellulosic polymer hydrogels are used in in-situ forming injectable scaffolds for bone regeneration. The hydrogels, however, generally suffer from their inherent weak mechanical properties. Calcium phosphate particles are used to enhance the mechanical strength and to improve the bone tissue regeneration capability of the scaffolds. In this study, hydroxypropyl methylcellulose (HPMC) was utilized as a polymeric matrix and β-tricalcium phosphate (β-TCP) and hydroxyapatite (HAp) nanoparticles were used to improve the mechanical properties. β-TCP Nanoparticles with plate-like morphology was synthesized through a wet chemical precipitation method. The morphology of the particles was confirmed using scanning electron microscope (SEM). Because the gelation temperature of the HPMC solution was above the temperature of normal human body, different concentrations of sodium sulphate (Na2SO4) were incorporated into the HPMC solutions to examine and adjust the gelation temperature. The cloud point and gelation temperature of the hydrogels were measured using UV/Vis spectroscopy and oscillatory rheometry, respectively. The injectability of the hydrogels, with different inorganic contents, was also measured using a universal testing machine. The results indicated that the cloud point and the gelation temperature of the hydrogels dropped with increase in the sodium sulphate concentration due to Hofmeister effect. The rheology measurements also revealed that β-tricalcium phosphate was more effective than hydroxyapatite in reducing the gelation temperature and enhancement of the modulus and viscosity of the hydrogels. The HPMC hydrogels containing β-tricalcium phosphate and hydroxyapatite nanoparticles were injectable at room temperature. The hydrogels based on HPMC matrix and the calcium phosphate nanoparticles provided promising hydrogels applicable as in-situ forming injectable scaffolds.