Stormwater drains are important sinks for microplastics and potentially aid the transport of microplastics from terrestrial to marine environments. Samples were collected from sediments in five stormwater drainage systems with different land uses, area, population, and sediment characteristics. Microplastic concentrations within the drainage network varied between 0 and 3500 microplastics per kg of dry sediment, with a mean concentration of 664 particles per kg. The majority of microplastics found were fibres. Polyethylene and polypropylene were the dominant polymer types (μ-FTIR). Based on a Generalized Linear Mixed Model, the expected number of microplastics for a representative urban drain was 760 particles per kg, with 245 particles per kg expected for an agricultural drain. Sediment grain size was a predictor for microplastic abundance, with sandier sediments containing more microplastics. This study is the first to quantify microplastic pollution in stormwater drainage systems in Western Australia.