Major target for prosthetic constructs of bone is the ability to elicit osteoinduction, osteoconduction and osteointegration. Promising constructs are represented by composite scaffolds made of biodegradable natural polysaccharides endowed with biocompatibility and mechanical properties. Alginate is a good candidate when its limits - weak mechanical properties and lack of cellular interactions - are overcome by reinforcing with inorganic components. In the present study an innovative strategy was developed to obtain three-dimensional scaffolds made up of alginate (Alg) and Hydroxyapatite (HAp). Scanning electron microscopy (SEM) observations, confocal laser scanning microscopy (CLSM) and micro-computed tomography (µ-CT) analysis of the scaffolds showed an optimal interconnected porous structure with pore sizes ranging between 100 µm and 300 µm and over 88% porosity. Proliferation assay and SEM observations demonstrated that human osteosarcoma cell lines seeded onto the scaffolds were able to proliferate, maintain osteoblast-like phenotype and massively colonize the structure. Overall, these combined results indicate that the novel alginate based composites efficiently support the adhesion and proliferation of cells showing at the same time adequate structural and physical-chemical properties for being used as scaffolds in bone tissue engineering strategies. Biocompatibility and bioactivity of the scaffolds were tested both in vitro and in vivo.