This work proposes a biorefinery approach for the exploitation of agri-food by-products, such as tomato pomace (TP), through the combination of mild chemical hydrolysis and high-pressure homogenization (HPH) in water not only to promote the recovery of cellulose but also its defibrillation to obtain nanocellulose. In particular, the cellulose pulp was isolated from TP using different combinations of chemical and physical processes, by applying HPH treatment (i) directly on the raw material, (ii) after the acid hydrolysis, and (iii) after alkaline hydrolysis. Moreover, the isolated cellulose was deconstructed to obtain cellulose nanoparticles, also through the application of the HPH treatment, enhancing the polymer properties. The structural and physical features of cellulose nanoparticles from TP were analyzed through Fourier-transform infrared spectroscopy (FT-IR) analysis, ?-potential measurement, and morphological analysis with SEM. The results clearly showed that the HPH treatment (80 MPa, 20 passes) at different stages of the process caused only a slight increase in the yield of cellulose recovery, but significantly contributed to obtaining defibrillated cellulose particles, characterized by smaller irregular domains containing elongated needle-like fibers.