This study investigates the influence of drainage conditions on the behaviour of monopile foundations embedded in sand. A three-dimensional coupled solid-fluid finite element model was developed to examine monopile response under drained, partially drained, and undrained conditions. The sand was modelled using the SANISAND-MS constitutive model to capture state-dependent and fabric-related effects, while the pile was treated as a rigid inclusion. The analysis focused on lateral load–displacement behaviour and pore pressure evolution during cyclic and monotonic loading. Results indicate that drainage conditions have a pronounced impact on soil resistance, stiffness degradation, and pore pressure response around the pile. The study provides valuable insight into the mechanisms governing monopile performance in sandy soils under different hydraulic conditions.