Under a simple redox system of selenite and ascorbic acid, we used Gracilaria lemaneiformis polysaccharides (GLPs) as a stabilizer and dispersing agent to generate well-dispersed and stable selenium nanoparticles (SeNPs). The size, stability, morphology and physicochemical properties of GLPs-SeNPs were characterized by dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Ultraviolet-visible spectroscopy (UV-vis), Fourier transform infrared (FT-IR) spectra, energy dispersive X-ray (EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Thermogravimetric (TG). The results showed that orange-red, amorphous, zero-valent and spherical GLPs-SeNPs with mean diameter of approximately 92.5 nm were successfully prepared, which exhibited good storage stability at 4 °C and remaining highly stable at different ion strengths and pH. The 2,2-diphenyl-1-pycrylhydrazyl (DPPH), 2,2'-Azinobis-(3-ethylbenzthiazoline-6-sulphonate) (ABTS) and superoxide anion radical (O2•-) radical scavenging ability of GLPs-SeNPs were higher than those of bare SeNPs, GLPs and sodium selenite (Na2SeO3), and could reach 103.41%, 94.23%, 86% at a concentration of 1.5 mg/mL, respectively. Besides, GLPs-SeNPs also showed higher inhibitory effects on α-amylase and α-glucosidase. In vitro cytotoxicity assay and hemolysis activity examinations indicated that GLPs-SeNPs have excellent biocompatibility. Therefore, the GLPs-SeNPs might be used as a potential antioxidant agent and antidiabetic agent for food and medical applications.Copyright © 2021 Elsevier B.V. All rights reserved.