The increasing application of glass fibre-reinforced polymer (GFRP) composites in structural systems requires reliable prediction of fatigue life and stiffness degradation under cyclic loading. This study experimentally investigates the fatigue behaviour of unidirectional S-glass/epoxy laminates fabricated using the hand lay-up process. Tension–tension fatigue tests were conducted at three stress ratios (R = 0.05, 0.1, and 0.2) and three maximum stress levels (0.6, 0.7, and 0.8 * UTS). Stress–life behaviour was evaluated using the Basquin and Goodman-corrected Basquin relations, while Kaplan–Meier statistics were employed to account for fatigue-life scatter and censored data. Residual stiffness degradation was characterised using four semi-empirical models (Yang, Mao, Wu, and Zong). Among these models, the Wu model showed the best agreement with the experimental data and accurately reproduced the nonlinear stiffness degradation behaviour. A generalized regression framework was further developed to correlate the degradation-model parameters with the stress ratio and normalized stress amplitude. The results demonstrate the feasibility of integrating experimental fatigue characterization, statistical reliability analysis, and semi-empirical modelling for the design-oriented fatigue assessment of GFRP laminates under cyclic loading.