Microplastics are persistent contaminants accumulating in the environment. Aquatic ecosystems have been studied worldwide, revealing ubiquitous contamination with microplastics. Microalgae, one of the most important primary producers in aquatic ecosystems, could suffer from microplastic contamination, leading to larger impacts on aquatic food webs. Nonetheless, little is known about the toxic effects of microplastics on microalgae populations. Thus, the objective of this review was to identify these effects and the impacts of microplastics on microalgae populations based on currently available literature, also identifying knowledge gaps. Even though microplastics seem to have limited effects on parameters such as growth, chlorophyll content, photosynthesis activity and reactive oxygen species (ROS), current environmental concentrations are not expected to induce toxicity. Even so, microplastics could disrupt population regulation mechanisms, by reducing the availability or absorption of nutrients (bottom-up) or reducing the population of predator species (top-down). Microplastics' properties can also influence the effects on microalgae, with smaller sizes and positive surface charges having higher toxicity. Therefore, more research is needed to better understand the effects of microplastics on microalgae, such as adaptation strategies, effects on population dynamics and microplastics properties influencing toxicity. Display omitted •Microplastics' current concentrations are not expected to directly harm microalgae.•Microplastics may inhibit predation or reduce nutrient availability in microalgae.•Factors responsible for species sensitivity and microplastics toxicity are unclear.