This study reports the development and characterization of highly porous ulvan-based beads synthesized via a double cross-linking strategy for application in wastewater treatment. Ulvan, a sulfated polysaccharide extracted from the green macroalgae Ulva fenestrata, was processed to form beads with enhanced structural integrity and surface properties. Physicochemical analyses confirmed a porous architecture, high sulfate group content, and a surface area of up to 70 m2/g. The adsorption performance of the beads was evaluated using cationic dyes. Adsorption kinetics, isotherms, and thermodynamic studies revealed high dye uptake capacities: 445.47 mg/g (Toluidine Blue O), 342.15 mg/g (Crystal Violet), and 368.76 mg/g (Methylene Blue). Kinetic modeling revealed a multistep adsorption mechanism, with external film diffusion as the rate-limiting step, followed by intraparticle diffusion and surface interactions driven by electrostatic attraction and hydrogen bonding. In addition, the beads exhibited clear selectivity toward cationic dyes and showed both charge and size-based discrimination. The findings highlight the effectiveness of the dual cross-linking approach in enhancing the adsorption performance of ulvan beads. This work presents a sustainable, bio-based material with dual selectivity for cationic dye removal, addressing the challenges of complex contaminant mixtures in wastewater. The integration of polymer chemistry and mesostructuring strategies provides a promising platform for the development of advanced adsorbents for environmental remediation.