AbstractOffshore wind turbine substructures consisting of cylindrical members are exposed to highly non-linear and breaking waves in shallow waters [1]. Those structures experience extreme impulsive loads of short duration from breaking waves that can cause permanent structural damage[2]. The main purpose of the present paper is to investigate the wave impact forces on a slender cylinder from plunging breaking waves in shallow waters both experimentally and numerically. The present study consists of two major parts: laboratory measurements and numerical simulations. The laboratory experiments are performed with regular waves. Plunging breaking waves are generated and free surface elevations are measured around the cylinder. Next, numerical simulations are carried out in the three-dimensional numerical wave tank REEF3D. The model is based on the incompressible Reynolds-averaged Navier-Stokes equations together with the k − ω for turbulence and the level set method for free surface. The numerical results are compared with the laboratory measurements in order to validate the numerical model. A good agreement between the computed results and the experimental data is seen for the breaking wave properties. Further, the breaking wave forces and the free surface deformations during the interaction of plunging breaking waves with a vertical cylinder are investigated and they are reasonably well represented in the numerical simulations.