Summary: Stabilized nanoparticles functionalized with carboxy groups were synthesized directly from dextran and acrylic acid (AA), without using any organic solvent and surfactant. The composition and morphology of these dextran‐based nanoparticles, as well as the mechanism of this one‐pot synthesis, were also investigated in this paper. This approach is anticipated to be applicable to various water‐soluble polysaccharides to fabricate nanoparticles facilely.