Recycling crushed waste oyster shells (WOS) as a fine aggregate is an attractive method of disposal. However, its use in geopolymer mortar has not been reported. The influence of PVA fibres on the engineering properties of the new geopolymer mortar is still unclear. To bridge the gap, this study investigated the influence of various PVA fibre contents (0–1.05 vol%) on the flowability, compressive, flexural strengths, drying shrinkage, sorptivity, chloride resistance, porosity, fibre dispersion, embodied CO₂ emissions (ECO₂e), and embodied energy (EE) of the geopolymer mortar. The results indicated that the inclusion of 0.15–1.05 vol% of PVA fibres improved the flexural strength by 10.10–42.31% and reduced the drying shrinkage by 13.37–65.79%. The flowability and compressive strength decreased by 10.78–34.28% and 7.50–27.65%, respectively, but they were sufficient for construction. The sorptivity increased by 1.45–15.16%, and the chloride resistance decreased by 15.09–56.35%, but the geopolymer mortar was still classified as low chloride penetrability. In summary, the optimal content of PVA fibres is 0.45 vol%, and the geopolymer mortar has good engineering properties and eco-efficiency. The cost analysis and high-temperature resistance of the geopolymer mortar are neglected in this study, which should be evaluated in future work.