In the present work, seafood by-products and derivates were exploited as raw materials to produce nanocrystalline calcium phosphates-based composites in light of the rising demand for waste recovery and valorisation. Mussel shells were transformed into hydroxyapatite by dissolution-precipitation synthesis at 45 °C, whereas chitosan from shrimp shells was introduced as a reinforcing biopolymer to produce hydroxyapatite/chitosan composites. The synthesised hydroxyapatite and hydroxyapatite/chitosan composite powders were cold sintered at room temperature under 1 GPa pressure for 10 min. The materials were consolidated up to ∼90% relative density and characterized mechanically. By increasing the polymer content up to 10 wt%, the flexural strength of the sintered pellets increases from ∼45 MPa to ∼57 MPa while the hardness decreases from ∼1.1 GPa to ∼0.8 GPa, thus better addressing the mechanical properties of cortical bone. Furthermore, hydroxyapatite/chitosan composites were proven to be bioactive, this demonstrating their potential use in bone tissue engineering applications.