Sodium alginate is a naturally occurring, biodegradable biopolymer that has been extensively utilized in the formulation of nanoparticles for controlled drug delivery systems. In this study, sodium alginate-based nanoparticles (NaAlgNPs) doped with Syringodium isoetifolium extract were synthesized and characterized to evaluate their antioxidant, anti-inflammatory, and anti-diabetic properties. Fourier-transform infrared (FT-IR) spectroscopy confirmed the presence of hydroxyl, ether, and carboxyl groups, thereby indicating robust interactions between sodium alginate and the seagrass extract. Field emission scanning electron microscopy (FESEM) revealed a well-defined morphology, while energy-dispersive X-ray (EDX) analysis validated the presence of bioactive elements. The antioxidant activity, assessed through 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assays, exhibited 60% and 65% radical scavenging at 100 µg/mL, respectively. The anti-inflammatory potential, evaluated through protein denaturation inhibition, demonstrated 80.25% inhibition at 50 µg/mL, exceeding that of Diclofenac sodium, which showed 77.17% inhibition. The anti-diabetic potential, determined via α-amylase and α-glucosidase inhibition assays, revealed 85% and 70% inhibition at 125 µg/mL, respectively, underscoring its significant hypoglycemic effects. These findings indicate that NaAlgNPs incorporated with Syringodium isoetifolium extract exhibit considerable bioactivity, positioning them as promising candidates for pharmaceutical applications. Future research should prioritize in vivo studies and clinical trials to assess biocompatibility and therapeutic efficacy. Furthermore, exploring these nanoparticles for targeted drug delivery, wound healing, and chronic disease management may further augment their biomedical potential, thereby supporting their utilization as eco-friendly and cost-effective therapeutic agents.