Soy sauce cake (SSC), a protein-rich by-product from soy sauce production, remains underutilized despite its potential for valorization into bioactive peptides. Halophilic bacteria, known for their robust enzymes, offer a sustainable solution for SSC biodegradation under high-salt conditions. This study investigates Mesobacillus sp. strain LC4, a protease-producing halophilic bacterium isolated from mangrove sediment. Phylogenetic and whole genome analyses confirmed strain LC4 as a novel Mesobacillus species, with ANIb and dDDH values below species delineation thresholds. Genome mining identified 25 extracellular protease-encoding genes, comprising serine and metalloproteases with physicochemical properties indicating thermotolerance and halophilicity. Crude protease assays demonstrated broad activity across temperatures (27-70 °C), pH levels (6-11), and salinities (0-15% w/v NaCl), with optimal activity at 40-60 °C, pH 8 and 4-8% NaCl. Enzymatic stability was retained in the presence of Ca2+ and Al3+ ions. Submerged fermentation of SSC by strain LC4 demonstrated significant protein hydrolysis (5.16 ± 0.18 mg/g dry sample) at 24 h, and peptide production (0.63 ± 0.03 mg/g dry sample) at 72 h. These findings have highlighted the potential of strain LC4 for upcycling proteinaceous high-salt food waste, providing a basis for future applications in sustainable food waste valorization and bioactive peptides production.