Several ground improvement techniques that are proven to be effective and economical solutions to increase the lateral stiffness and strength of weak soils around piles often result in unwarranted conservative volumes of soil improvement. There are also no rigorous techniques to analyze seismic behavior of piles in improved soils that can be utilized in day-to-day engineering practice. In this study, a stand-alone finite element computer code called DYPAC (Dynamic Piles Analysis Code) using the Beams on Non-linear Winkler Foundation (BNWF) approach is developed. DYPAC analyzes the seismic response of a single pile in improved and unimproved soils. This computer code models the pile as a beam element and the non-linear soil behavior as springs and viscous dashpots using a non-linear p-y element, where y is the pile displacement and p is the soil reaction per unit length of the pile. This non-linear p-y element accounts for soil yielding, gapping, radiation damping, and soil cave-in and recompression during seismic loading simulations. A method to modify the p-y curves to account for limited lateral extent of ground improvement is proposed and validated. The input parameters for these curves can directly be obtained from in-situ or laboratory soil tests. These p-y curves were input in to DYPAC to analyze a series of dynamic centrifuge tests of single piles in soils improved using Cement Deep Soil Mixing (CDSM). Free-field site response analyses were performed using the DEEPSOIL computer program and the soil displacement-time histories were input to the free-field ends of the non-linear p-y elements. The predictions made by DYPAC are validated using the centrifuge test results.