Bio-based and biodegradable polymers are essential for a sustainable society. Cellulose is the most abundant biopolymer on earth; however, derivatization is necessary for its processing, which slows down its biodegradability dramatically, e.g. used cigarette filters made from cellulose acetate are barely biodegradable. We developed the first reversible modification of cellulose, which allows processing and guarantees full biodegradation even at high degrees of substitution as the linkers, acetals, can be cleaved first during the degradation process releasing native cellulose that biodegrades in a second step. Acetalization is a versatile platform approach to bio-based and fully degradable cellulose-derivatives, which are characterized by solubility in common organic solvents (alcohols, aromatic and chlorinated solvents), adjustable glass transition temperatures (-48 °C < T_g < 80 °C), young's modulus (1.9 MPa < E < 58 MPa) and contact angle (86°< θ < 124°). In contrast to previously known cellulose modifications, cellulose acetals remain fully degradable as the acetal bond is reversible and undergoes an acidic cleavage under desired conditions, for instance in compost, followed by enzymatic degradation of the remaining cellulose backbone. With climate change and plastic pollution, these new and versatile cellulose acetals provide bio-based and biodegradable alternatives to fossil-based and non-degradable polyolefin plastics, leading to a more sustainable future for our planet.