Phosphoric acid is widely used for the swelling and hydrolysis of cellulose. The detailed description of molecular interactions between cellulose and phosphoric acid is essential for understanding and controlling these processes. Here, to obtain structural insights into the swelling behavior, we investigated the structural evolution of cellulose swollen in concentrated phosphoric acid solution using X-ray fiber diffraction and solid-state NMR spectroscopy. We observed the formation of a crystalline complex of cellulose and phosphoric acid at − 40 °C, where cellulose molecules adopt a seven-fold helical conformation. This structure is the second known cellulose-acid crystalline complex and the first cellulosic crystal consisting of seven-fold helical chains. Our observation highlights the conformational flexibility of cellulose molecules in the solvated states and the strong influence of cellulose-acid interactions on the packing and conformation of cellulose molecules.