Open ocean aquaculture cages became a promising alternative to traditional fish cage designs recently. The offshore environment implies larger loads on the structure and thus higher risk for fish loss. Floating rigid aquaculture cages with stiff nets are considered as a possible solution to cope with these new challenges. Their design process requires more advanced tools to account for the non-linear fluid–structure interaction. This paper presents a suitable numerical approach for analyzing the interaction of offshore aquaculture cages and waves using computational fluid dynamics. Here, a numerical wave tank accounts for the accurate propagation of the waves, and dynamic solutions are utilized for the cage system. Two-way coupling is enabled by accounting for the influence of the net on the fluid. The numerical model is validated against measurements for the loads on and the responses of a mobile floating fish farm in waves and current.