The present work provides the first records on microplastic (MP) amounts and types in rivers and wastewater effluents entering the Aegean Sea, eastern Mediterranean. Two rivers were sampled using a manta net (mesh size, 0.33 mm): a small urban and a medium-sized river with a rural, semiurban catchment. MPs in wastewater samples were collected at two wastewater treatment plants (WWTPs) within the Athens metropolitan area after secondary treatment and from a pilot biological membrane unit (MBR), using a series of sieves. MPs in the samples were identified using stereoscopic image analysis and spectroscopic techniques. MP concentrations in the rivers were found to be variable, with as high as 27.73 items m−3 in the urban river. Differences in MP shape types, sizes, and polymer types reflect catchment size and usage. MP concentration in wastewater effluents was found to be 100 times higher in the secondary treatment (213 items m−3) than that in the pilot MBR (2.29 items m−3), with filaments and polymers indicative of synthetic textiles and household use. Further research is needed in order to accurately determine variability in MP concentrations and fluxes from these two pathways in the eastern Mediterranean Sea and elucidate the role of rivers in MP retention.