Large quantities of microplastics are thought to be emitted to freshwater environments via wastewater treatment plants (WWTPs). To evaluate the occurrence of microplastics in Korean WWTPs, a nationwide study was conducted for the first time in 50 representative WWTPs with large treatment capacities. Grab sampling and laboratory filtration were used for influents, whereas in situ filtration using a custom-made sampling device was used for effluents. The filtrates were pretreated using wet peroxidation and density separation prior to the identification of microplastics with a dissection microscope and Fourier-transform infrared spectroscopy. Pooled analyses of the microplastics revealed that they were predominantly fragment-shaped, and thermoplastics and synthetic fibers were the dominant microplastic materials in WWTPs. The concentration ranged from 10 to 470 L in influents and 0.004 to 0.51 L in effluents. The removal efficiency of microplastics during wastewater treatment was calculated as 98.7-99.99% in 31 WWTPs. Additionally, WWTPs using advanced phosphorus removal processes exhibited higher removal efficiency than those not implementing such processes. Power law distribution was successful in describing microplastic particle sizes down to 100 μm, although it was not applicable for smaller particles. This comprehensive monitoring study provides information on the current level and characteristics of microplastics in WWTPs in Korea.