Abstract: With the continuous input of microplastics into freshwater environments, their long-term coexistence with cyanobacterial blooms in eutrophic waters has gradually formed a combined pollution system involving multiple interacting pathways. Microplastics can influence cyanobacterial growth, aggregation behavior, and bloom formation by providing attachment substrates, enriching nutrients, and altering the light environment. In addition, microplastics as well as their leachates and adsorbed contaminants may induce oxidative stress responses in cyanobacteria, thereby regulating the synthesis and release of cyanotoxins. Conversely, during cyanobacterial blooms, large amounts of extracellular polymeric substances (EPS) released and the biofilms formed can significantly modify the surface properties, aggregation state, and sedimentation behavior of microplastics, enhance their capacity to adsorb pollutants, and alter their environmental fate in the water column and sediments. The resulting microplastic–cyanobacteria heteroaggregates may further amplify ecological risks by expanding bloom magnitude and prolonging bloom duration, facilitating the transfer of contaminants through food webs, and disturbing biogeochemical cycling processes. Based on existing studies, the bidirectional interactions between microplastics and cyanobacterial blooms at the levels of physiological responses, population behaviors, and ecological processes are systematically summarized, with particular emphasis on their amplifying effects on cyanobacterial blooms and ecological risks, providing a theoretical basis for the risk assessment and management of combined pollution in eutrophic freshwater systems.