Offshore monopile foundations are widely used in marine and offshore engineering, particularly for wind energy applications, where they are subjected to severe environmental loads and may undergo large deflections. In this respect, linear or weakly nonlinear analyses often prove inadequate, especially under resonant conditions [2] and for impacting wave loads [3], while fully three-dimensional solid models remain prohibitive for long-term dynamic simulations or parametric studies [4]. With a design focus on operational limits and fatigue-induced vulnerability, accurate prediction of the dynamic response under wave excitation therefore requires numerical models capable of handling geometric nonlinearities while preserving computational efficiency [3]. In this contribution, a geometrically nonlinear time-domain analysis of an offshore monopile structure, embedded in the seabed and exposed to wave loading, is presented. The proposed methodology extends the static formulation of the state-of-the-art nonlinear spatial Bernoulli beam by Bauer et al. [1] to dynamic scenarios, addressing the key knowledge gap related to the development of high-fidelity reduced-dimensional models for the dynamics of slender offshore structures. Large displacements and rotations are consistently treated within a geometrically exact kinematic framework. The isogeometric analysis paradigm [5] is employed, whereby enhanced prediction accuracy of the seakeeping behavior is enabled via a continuous-curvature geometric description endowed with a reduced number of degrees of freedom as a result of a problem-tailored selection of the interelement continuity in a structured spline-based discretization. The study unveils the potential of isogeometric nonlinear beam models for the dynamics of slender offshore structures, paving the way for new perspectives in nonlinear offshore engineering simulations relative to standard finite element approaches.