AbstractSilica-based optical fibres are a workhorse of nonlinear optics, providing ready access to a range of nonlinear phenomena including solitons and self-phase modulation. However, they have one fundamental limitation: due to the amorphous nature of silica, they do not exhibit second-order nonlinearity, except for negligible contributions from surfaces. Here we demonstrate second-harmonic generation in functionalized optical fibres by using a monolayer of highly nonlinear MoS₂ directly grown on the fibre’s core. The MoS₂-functionalized fibre exhibits a second-order susceptibility (χ⁽²⁾) value of 44 pm V^–1 and a second-harmonic generation conversion efficiency of 0.2 × 10^–3 m⁻² W⁻¹. This approach is scalable and can be generalized to other transition metal dichalcogenides and a wide range of waveguide systems. Our results demonstrate a new approach towards efficient in-fibre second-harmonic generation sources and may establish a platform for χ⁽²⁾-based nonlinear fibre optics, optoelectronics, photonics platforms, integrated optical architectures and active fibre networks.