Basalt fibres are increasingly employed as reinforcements in marine composites, but their behaviour in natural marine environments is underexplored. This study investigates basalt fibre ageing in renewed natural seawater at 15 °C and 40 °C. After one month in seawater at 15 °C and 40 °C, tensile strength dropped, stabilizing at approximately −40% and −60%, respectively. This rapid initial property decline, followed by slower degradation, is attributed to an altered surface layer on the fibres. Initially causing significant property loss, this layer then plays a protective role, preserving the fibre core. The impact on basalt/epoxy composites exposed to 7.5 years of seawater was less severe, with a 20% loss at 40 °C, demonstrating the protective function of the matrix. This study suggests that basalt fibres undergo rapid, then stable, property degradation in water, but remain suitable for use as epoxy matrix composite reinforcements, thanks to the protective role of the resin.