Algae are considered a valuable source of polysaccharides with important bioactive characteristics that benefit human health, such as anti-tumor, antioxidant, and antiviral properties. Despite the recognized importance of these organisms, microalgae have been virtually unexplored relative to macroalgae. Chaetoceros muelleri is a cosmopolitan planktonic diatom microalga present in the Sea of ​​Cortez. Its biomass is essential in aquaculture as it is a source of high-value products (long-chain polyunsaturated fatty acids, proteins, and carbohydrates). Several studies on sulfated exopolysaccharides (sEPS) from macroalgae have been reported; however, information on sEPS from microalgae is scarce. This research aims to generate knowledge about the macromolecular characteristics and the gelling and bioactive properties of sEPS from Chaetoceros muelleri. In the present study, the culture of this diatom was established under controlled laboratory conditions. From the biomass obtained, the extraction of sEPS was carried out, bringing a yield of 2.3% (w/w of dry biomass). The sEPS were analyzed by Fourier transform infrared spectroscopy showing characteristic bands reported for these macromolecules in the range of 3405 to 821 cm^-1, thus confirming their molecular identity. High-performance liquid chromatography coupled to a dynamic light scattering detector registered a molecular weight (Mw), polydispersity index (PI), intrinsic viscosity [η], radius of gyration (RG), and hydrodynamic radius (Rh) value of 945 kDa, 1.14, 653 mL/g, 69 nm, and 45 nm, respectively for sEPS. In addition, K and α constants in sEPS were 1.112 x 10^-2 and 0.817, respectively, suggesting a molecular random coil structure in these polysaccharides.