In this study, the fatigue behaviour of sandwich composites was investigated in both, air and seawater. S–N curves were obtained for different stress levels. 3-point bending (3PB) tests were also performed to investigate strength degradation during the fatigue test. Finite element analyses were carried out to determine the stress distribution in the test coupons used in bending tests. These were used to predict the failure of the composite and to interpret the experimental results. There is good agreement between finite element analysis and experimental results at lower loads. Small deviations were observed at higher loads due to the influence of defects. The typical failure modes observed under quasi-static 3PBT were debonding, fibre pull-out and matrix cracking.