The effect of moisture absorption on the risk of interface delamination in a fan-out (FO) wafer level package (WLP) was investigated by using experimental and numerical analyses. A double-cantilever-beam (DCB) fracture mechanics test was applied to quantify the degradation of interfacial debonding resistance due to moisture exposure. The driving forces of delamination on various interfaces in the FO WLP were evaluated by using a coupled hygro-thermo-mechanical model. The moisture diffusion related material models such as diffusivity, saturation concentration and the hygroscopic swelling of the polymer constituents in the FO package were characterized experimentally and implemented in the numerical stress model. The numerical model was applied to evaluate the transient stress evolutions at critical locations around the corners of the Si die during moisture sensitivity tests. It was observed that the moisture absorption degrades adhesion significantly. It also leads to a change in stress state around the corners of Si die from compression to tension, and as a result, significantly increase the risk of delamination.