To mitigate marine pollution from single-use plastics, it is imperative to transition to next-generation commodity materials that are biomass-derived, recyclable, and marine biodegradable in the event of accidental ocean release. Here, we report the development of an optically transparent, millimeter-thick paperboard—termed transparent paperboard (tPB)—produced via the dissolution and coagulation of cellulose. The tPB is composed entirely of pristine cellulose and is compositionally identical to conventional paper. A cup-shaped tPB can contain just-boiled water without requiring an internal film coating, owing to its high wet tensile strength and anisotropic thermal properties. Moreover, spent tPB is fully recyclable within a closed-loop system in which all chemicals and water are also recovered. On-site degradation tests and metagenomic analyses further confirm the marine biodegradability of tPB across environments ranging from shallow waters to abyssal depths. Consequently, tPB is expected to serve as a fully circular commodity material, pivotal for sustainable societies of the future.