Microplastics, ubiquitous contaminants in the marine environment, pose ecological threats to marine organisms. Ocean currents play a pivotal role in dispersing these particles, influencing their pathways and fate in the ocean. This study explores the effect of water mass dispersal on the transport trajectories of microplastics. We combine numerical simulations using a high-resolution ocean model with observational data to track the movement of microplastics over extensive spatial and temporal scales. We demonstrate that the transport pathways of microplastics are not solely determined by surface currents but are significantly influenced by secondary circulation patterns induced by water mass interactions. These insights elucidate the intricate interplay between physical oceanographic processes and microplastic transport, advancing our understanding of the mechanisms governing the transport and accumulation of microplastics in the ocean.