Inspired by mussels and sandcastle worms, a water‐soluble adhesive is prepared by grafting catechol and phosphoric acid functionalities to the polyoxetane backbone, which achieves improved adhesion in humid environment. The adhesive is characterized by ¹H and ³¹P NMR spectroscopy and size‐exclusive chromatography. The influence of the content ratio of catechol and different phosphoric acid groups and the Fe³⁺ ions as the crosslinker on the outcome of the adhesive properties is investigated. When the molar ratio of Fe³⁺ to catechol and PO₄ is 1/1 and 0.7/1, respectively, the best bonding strength of 0.35 MPa is achieved for the adhesive containing 5 mol% catechol and 26 mol% bis‐phosphoric acid groups under humid conditions. It is found out that Fe³⁺ interacts not only with the catechol, but also with the bis‐phosphoric acid groups, which accounts for its performance in humid conditions.