Bioactive gelatin–graphene oxide (GO) nanocomposites with varying GO contents were fabricated by a solution-casting method. With the addition of 1 wt% GO, the tensile strength, Young's modulus and energy at break of gelatin were found to increase by 84%, 65% and 158%, respectively. Such reinforcement effects were investigated in detail in terms of the size and morphology of GO sheets, the dispersion degree of GO sheets in gelatin matrix and the interactions between the two phases. The Young's moduli of the nanocomposites were well predicted by the Mori–Tanaka model with application of the effective volume fraction of reinforcement that takes into account the nanofiller and a fraction of the macromolecules adsorbed onto its surface. The GO nanosheets also improved the bioactivity of gelatin by inducing more calcium phosphate nanocrystals on the composites. GO acts as both an effective reinforcement filler and a biological activator in hydrophilic biopolymers such as gelatin, offering the biopolymer–GO nanocomposites great potential to be further developed in biomedical fields.