<p>Plastic foams play a crucial role across various industries and building constructions, due to their lightweight structure, thermal insulation properties, and energy absorption capabilities. However, the escalating global demand for petrochemical-based foams is raising significant environmental concerns. Here, we report an all-cellulose molecular foam through an ethanol-induced cellulose molecular programmed assembly. This cellulose molecular foam features a honeycomb-like gradient porous structure, exhibits a high compressive modulus of 11.8 MPa, demonstrates a high thermal stability up to 264.1 °C, and maintains a&#xa0;low thermal conductivity of 0.047 W m⁻¹ K⁻¹. Additionally, it supports diverse shaping processes including casting, molding, and continuous manufacturing. Due to its molecular-level reversible design, all-cellulose foam is both recyclable and biodegradable, offering a potential substitute for conventional petrochemical foams in numerous building and industrial applications. Furthermore, a life cycle assessment reveals that all-cellulose foam significantly reduces carbon emissions, affirming its environmental benefits and positioning it as a promising, eco-friendly alternative.</p>