Cellulosic nanofibrils (CNFs) were isolated from one of the most widespread freshwater macroalgae, Aegagropila linnaei. The algae were first carboxylated with a recyclable dicarboxylic acid, which facilitated deconstruction into CNFs via microfluidization while preserving the protein component. For comparison, cellulosic fibrils were also isolated by chemical treatment of the algae with sodium chlorite. Compared with the energy demanded for deconstruction of wood fibers, algal biomass required substantially lower levels. Nevertheless, the resultant nanofibrils were more crystalline (crystallinity index > 90%) and had a higher degree of polymerization (DP > 2500). Taking advantage of these properties, algal CNFs were used to produce films or nanopapers (tensile strength of up to 120 MPa), the strength of which resulted from protein-enhanced interfibrillar adhesion. Besides being translucent and flexible, the nanopapers displayed unusually high thermal stability (up to 349 °C). Overall, we demonstrate a high-end utilization of a renewable bioresource that is available in large volumes, for example, in the form of algal blooms.