BACKGROUNDDiclofenac (DCF), a widely used nonsteroidal anti‐inflammatory drug, has been detected in the environment. Its presence is of concern due to potential ecological impacts and health risks. The development of effective technologies for the removal of DCF from water is of paramount importance. The efficient removal of DCF from highly diluted solutions poses a further significant challenge. Microcystis aeruginosa, a rapidly growing cyanobacterium, possesses significant potential as a valuable biomass resource that is currently underutilized. This study investigates various pretreatments to enhance the adsorption capacity of nontoxic M. aeruginosa ACCMU‐118 biomass for DCF removal.RESULTSAlkali‐pretreated biomass achieved over 90% DCF removal at concentrations of 1000–5000 μg L−1. The modified biomass showed comparable efficiency in removing DCF from binary solutions containing heavy metals (Cd2+) or ibuprofen. Fourier transform infrared analysis revealed hydroxyl, carboxylic acid and alkene functional groups on the biomass surface contributing to DCF sorption. Chemical pretreatment enhanced binding site exposure, increasing DCF sorption capacity. Thermodynamic analysis confirmed the exothermic sorption and thermodynamically favorable process. Methanol regeneration allowed multiple sorption–desorption cycles, enhancing sustainability.CONCLUSIONThis cost‐effective approach demonstrates the potential of M. aeruginosa biomass for selective and efficient DCF removal from contaminated water sources. Chemical pretreatment expands the application potential of M. aeruginosa biomass as a biosorbent, showcasing its advantages in addressing the challenges of contaminated water treatment. © 2023 Society of Chemical Industry.