<p>Petroleum-based plastics have raised global concerns in recent years. Degradable bio-based supramolecular plastics (SPs), coupled with eco-friendly hydroplastic processing, become sustainable alternatives. However, the inherent contradiction between hydroplastic processability and water-resistance restricts their applications in humid environments. Here, we report a cascade strategy of pre-hydroplastic processing and post-noncovalent crosslinking to stepwise achieve hydroplastic processing and water-resistance of alginate SPs. Alginate-cetyltrimethylammonium (SA-CTAB) SPs are firstly processed into various two-dimensional/three-dimensional shapes due to the excellent hydroplasticity. Then, Ca²⁺ ions are introduced to improve the mechanical strength, water-resistance, and flame retardancy by forming robust crosslinks with alginate chains. The wet tensile strength and Young’s modulus of the obtained SA-CTAB-Ca²⁺ SPs are respectively enhanced for 38 times to 15 MPa and 110 times to 0.57 GPa after crosslinking, which will substantially broaden their applicability in humid environments. We expect this work will provide a sustainable processing route for high-performance alginate SPs.</p>