Abstract: Subsea pipelines, as key infrastructure for offshore oil and gas transport, inevitably cross irregular seabed terrain due to their long-span, slender configuration. Existing international design codes and most prior studies largely idealize the seabed as flat or uniformly sloping, and therefore overlook the effects of geomorphic features such as sand waves. Yet, the presence of sand waves fundamentally alters pipe-soil contact conditions and boundary constraints, affecting lateral soil resistance and creating uncertainties in pipeline design. This study systematically investigates the lateral soil resistance of subsea pipelines through model tests examining various configurations of sand wave heights, lengths, and pipe-wave spacings alongside a flat seabed control. The experimental programme focused on the pipe-soil interaction response curves, the breakout resistance, and the resulting pipeline motion modes. Using Particle Image Velocimetry (PIV), we extracted soil particle displacement vectors, continuous displacement and shear strain contour maps during pipeline penetration, revealing soil kinematics, slip surface evolution, failure angles, and the corresponding kinematic zones. It is found that different from the DNV three-stage lateral resistance model for flat sandy seabeds, the pipeline lateral response on flat and sand-wave seabeds exhibits a two-stage characteristic without an obvious strain-softening stage under dry sand and fully drained saturated sand conditions. However, sand waves markedly alter failure mechanisms: the single breakout mode observed on flat seabeds transitions into crest breakout, intra-sand-wave penetration, and sand-wave overpassing on sand-wave terrain; variations in sand wave parameters notably affect the failure angle and passive resistance coefficients of the soil, impacting the breakout resistance. These findings provide a mechanistic basis for more reliable stability assessment and improved design of subsea pipelines in complex sand-wave seabed environments.