Organic redox-reactive polymers have garnered great attention as a promising alternative for conventional transition-metal compounds in sodium-ion batteries (NIBs) due to their low cost, structural flexibility and diverse structure. Among this class of materials, polyimides with high mechanical strength, excellent thermal stability and high density of electroactive functional groups have shown promise as low-cost electrode materials for NIBs. Herein, a simple hydrothermal method was used to synthesise pyromellitic dianhydride-based polyimides [C16H6O4N2]n. The polyimides consisting of interconnected nanosheets with a microflower-like morphology were tested as an NIB anode. The polyimide electrode exhibited a stable discharge capacity of 125 mAh g−1 at a current density of 25 mA g−1 at the 100th cycle. At a high current density of 2 A g−1, the electrode delivered a discharge capacity of 43 mAh g−1. The capacity contribution of this polyimide electrode mainly occurred below 1.5 V making it suitable as an organic NIB anode. The mechanism of sodiation and desodiation during discharge and charge was studied using Fourier transform infrared spectroscopy, in which this polyimide experienced two-step enolisation reaction with reversible insertion of two sodium ions during the redox electrochemical reaction.