Hydroxyapatite (HAp) is a widely used bioceramic in bone tissue engineering due to its biocompatibility and osteoinductive capacity; however, sustainable low-cost synthesis routes remain a challenge. This study evaluated HAp-polysaccharide composite synthesis from nejayote, the alkaline wastewater of maize nixtamalization, via chemical precipitation with (NH₄)₃PO₄ under controlled and uncontrolled pH, followed by calcination at 550 °C for 2 or 4 h. Controlled pH synthesis yielded higher solid recovery (89.8% vs. 76.4%), better calcium removal (99.8% vs. 87.4%), and smaller particle sizes (423.6 nm vs. 715.0 nm). XRD and FTIR confirmed HAp formation in both conditions, with crystallinity increasing upon calcination. Monomeric composition analysis revealed co-precipitation of amylose and arabinoxylan-derived polysaccharides in uncalcined samples, progressively eliminated by thermal treatment. Cell viability assays with human fetal osteoblasts (hFOB 1.19) confirmed non-cytotoxicity at all concentrations tested (10–633 μg/mL). Uncalcined composites synthesized without pH control achieved 126% cell viability at 633 μg/mL, surpassing pH-controlled and calcined counterparts (90–100%), suggesting active promotion of osteoblast proliferation, further supported by fluorescence imaging. These results establish nejayote as a viable dual source of calcium and polysaccharides for sustainable HAp composite synthesis with biomedical potential.