Development of sustainable fibres for high performance applications is challenging because of the high mechanical strength properties demanded by such fibres and the general lack of such properties in natural fibres. The mechanical properties of Himalayan giant nettle (Girardinia diversifolia L.) fibre are measured and compared to those for European nettle (Urtica dioica L.) fibre. The mean length of G. diversifolia fibre is substantially higher than U. dioica fibre, and the longest fibre length reported for any bast fibre. G. diversifolia and U. dioica fibres have similar cross-sectional area, but G. diversifolia fibre is a wider, flatter, oval shape with a significantly wider lumen. G. diversifolia fibres display a generally linear stress/strain curve. Ultimate stress value for G. diversifolia fibre is over twice that of U. dioica fibre, and the highest of any bast fibre reported. G. diversifolia fibre also displays greatest strain at failure of any bast fibre. Young's modulus for G. diversifolia and U. dioica fibre are similar. Himalayan giant nettle fibres display tensile properties that offer potential in high performance applications.