Dialdehyde sodium alginate (DSA) is an eco-crosslinker attracting extensive interest while undergoing limited large-scale applications. Herein, we employed DSA to react with dicyandiamide (DA) for engineering a biomass-derived retanning agent (BDR) towards addressing the long-term toxicity of residual formaldehyde (FA) in leather caused by amino resins. Results confirmed that BDR reserved the structural features of DSA by grafting DA onto DSA molecules. Owing to the suitable molecular weight (main components, 1424–1462?g/mol) and abundant oxygen-containing groups of BDR endowed by DSA, BDR-treated chrome-free leather showed higher hydrothermal stability (82.4?°C), thickening ratio (13.93?%), mechanical strengths (17.2?N/mm² for tensile strength and 120?N/mm for tear strength), and fullness compared with industrial dicyandiamide-FA-resin (DFR)-treated leather. The FA-free feature of DSA led to BDR-treated leather containing no FA, while FA in DFR-treated leather reached 591.5?mg/kg. This work provided new insights into broadening the large-scale application scopes of DSA.