Cellulose fibre (CF) has been widely employed as an adsorbent because of its effective capacity for adsorbing toxic components of cigarette smoke. There is a lack of studies of CF adsorbing pharmaceuticals from water. Herein, we evaluated the adsorption of potassium diclofenac (PD) from an aqueous medium by CF. In addition, we developed and prepared CF covered with polypyrrole (PPy) to improve the adsorptive capacity for removal of PD. From the infrared characterization, the main bands related to the functional groups of each component, PPy and CF, were identified; in thermogravimetry it was observed that both the CF and composite support relatively high temperatures and in the scanning electron microscopy it was possible to observe that the composite has a fibrous structure related to CF in which PPy particles are aggregated. In adsorption studies of PD in an aqueous medium, the pH, the contact time between the material and analyte and equilibrium concentration were evaluated. The first study carried out was the influence of pH on the adsorbed amount of PD, the optimum pH chosen for the experiments being 2.0 and 6.0 for CF and PPy/CF, respectively. In the kinetic study, adsorption occurred rapidly and equilibrium times were achieved within 30min (CF) and 15min (PPy/CF). The kinetic data were analysed using four models: pseudo-first-order, pseudo-second-order, Elovich and intraparticle diffusion and the adsorption kinetics were best described by the pseudo-second-order model with R2=0.999 for both materials, indicating that a chemical adsorption occurred. The Langmuir, Freundlich, Sips, single-site Langmuir–Freundlich and dual-site Langmuir–Freundlich models were used to analyse the adsorption equilibrium data obtained by the variation of the PD concentration in the aqueous solution and the dual-site Langmuir–Freundlich model presented the best fits, CF: R2=0.982 and PPy/CF: R2=0.997, indicating that there are two types of sites available for adsorption, homogeneous and heterogeneous, in the assessed materials. In addition, the composite had the best maximum adsorptive capacity, Q=210.07mgg−1, while the CF presented Q=50.33mgg−1. Herein, we presented the preparation of cellulose fibre (CF) decorated with polypyrrole (PPy) as an efficient adsorbent for removal of potassium diclofenac (PD) from aqueous medium, which can be used in separation process as solid phase extraction and chromatographic and electrophoretic techniques. The kinetics and isotherms studies showed a great potential of this material for this purpose. The PPy/CF were prepared via in situ polymerization and characterized by FTIR, TG and SEM. Display omitted •New composite adsorbent (PPy/CF) was easily synthesized and well characterized.•PPy/CF was efficient for removal of PD from aqueous medium.•Adsorption studies allowed understanding the adsorption mechanism of PD onto PPy/CF.•PPy/CF showed excellent adsorption capacity compared to pure CF.•PPy/CF presented good stability over a wide pH range (pH2–10).