π–π interactions between graphene and organic dyes with a conjugate aromatic structure play a key role in the field of high-efficiency, broad-spectrum adsorbents for the removal of water contaminants. L-Cysteine reduced graphene oxide (RGO-Cys) has a good conjugate structure and dispersity in aqueous solution, endowing it with great adsorption efficiency towards anionic, nonionic and cationic dyes with a conjugate aromatic structure mainly via π–π interactions, as proved by the Raman spectrum and special adsorption experiments. The maximum adsorption capability for anionic indigo carmine (IC) and cationic neutral red (NR) is as high as 1005.7 mg g⁻¹ and 1301.8 mg g⁻¹, respectively, the former being the highest among those reported for adsorbents known to date. The total adsorption amount in mixed dye solutions is even higher (>3500 mg g⁻¹), the highest total capability for simultaneous adsorption of anionic and cationic dyes in their solution mixtures. The π–π stacking adsorption mechanism ensures RGO-Cys to be used as a broad-spectrum adsorbent with high efficiency for many kinds of dye contaminants in water while the remnant carboxyl groups on graphene nanosheets facilitate effective adsorption towards Cu²⁺ with a capability as high as 139.2 mg g⁻¹, opening up many possibilities for the use of graphene in water cleaning including disinfection, decontamination, and desalination.