Tsunamis are long period, large amplitude waves of tectonic origin that cause serious havoc to the life and infrastructures as it hits the coast. Sea dikes are nowadays adopted for the protection of onshore infrastructures against devastating tsunami wave. A well tested mathematical model is used to bring out important counter intuitive results on the aspect of reducing tsunami impact forces on on-shore buildings using sea dikes. The mathematical model is based on numerical solution of the general Reynolds Averaged Navier Stokes (RANS) equations, with SST–k–ω model for turbulence closure. Tsunami waves are allowed to propagate on a 1 in 40 sloped beach and overtop a continuous sea dike of trapezoidal cross section before impacting on buildings. The combined effect of various parameters such as position of the dike and shape and size of the dike has been studied, for sloping beaches. A sea dike placed far from the building may increase the tsunami impact force, if its size is not appropriate. It is found that a sea dike positioned close to the building offers best protection only if the height of the wave is small. If the wave height is large, there is an optimal distance at which the dike should be located for minimizing the tsunami impact force. Placing a second sea dike in addition to an existing sea dike may not reduce the tsunami impact force, if the location and size of the sea dikes are not appropriate. For intermittent / discontinuous sea dikes, the flow fields are 3D in nature, making it necessary for three-dimensional model studies to analyze their effects on the peak tsunami impact force on a downstream structure. The simulation results for different dike length to gap ratios have shown that sea dikes with 25% gaps will be adequate as the peak impact force on an onshore building is reduced by 36%, while a continuous sea dike can reduce the peak impact force by 40%. A fifty-fifty composite dike wherein a larger trapezoidal cross-section is alternated with a smaller rectangular section is capable of reducing peak tsunami impact force by approximately 32%. Two staggered rows of intermittent dikes give better protection than a single row of intermittent dike. However, the protection is marginally lesser than that given by a single continuous dike.