In the UK, microplastics are considered an emergent contaminant but not pollutants and, at this moment, are not monitored as part of water quality tests. As a result, microplastic abundance within many UK rivers is unknown. This also means that the many factors influencing microplastic abundances within individual rivers, such as seasonality and rainfall, are currently understudied. This study focused on the seasonal impacts, including the effect of rainfall, on microplastic abundance within the surface water of the River Thames. Ten sites in eight areas were sampled along the tidal section of the river, starting at Teddington and ending in Southend-on-Sea. Three litres of surface water were collected monthly at high tide from land-based infrastructure from May 2019 to May 2021. A total of 6657 pieces were identified and recorded throughout this study. However, there was no significant variation between seasons and microplastic abundance, with the mean microplastic abundance along the River Thames ranging from 10.29 ± 4.34 L−1 (spring) to 14.83 ± 7.49 L−1 (autumn). However, there was a substantial difference between MP abundance observed between consecutive seasons in 2019–2021. Fibres were the most abundant throughout, making up 77.1–85.96% of samples in all four seasons. A total of 1041 pieces of suspected microplastic were analysed via Fourier-transform infrared radiation (IR) spectroscopy, of which 176 pieces were not identified. Out of the 41 different polymers identified, the most common were polyvinyl chloride (PVC) (24.5%), polystyrene (PS) (9.8%) and polychloroprene (PCP) (7.69%). Polychloroprene was less observed in the spring months compared to other seasons. This study demonstrates a yearly variation in seasonal microplastic abundance with less MP observed in 2020, suggesting other factors, such as COVID-19, have a more substantial impact on microplastic abundance than environmental factors.