Increase in the contamination of the aquatic environments is a global challenge; hence, understanding the sources of priority substances (PSs) is essential. In an attempt to implement this principle, a year-long monitoring covering all seasons was carried out in the influents and effluents of four largest wastewater treatment plants (WWTPs) in Istanbul. Results obtained showed the presence of 48 PSs (66% of the target compounds) including pesticides, polycyclic aromatic hydrocarbons (PAHs), volatile organic compounds (VOCs), dioxins and dioxin-like compounds (DLCs), alkylphenols, phthalates, and metals ranging from low nanograms to micrograms per liter. Priority hazardous substances that were banned for long were still found to be present in wastewaters. PAHs, DLCs, alkylphenols, and metals were found to be present in all samples. Di(2-ethylhexyl) phthalate (DEHP) and DLCs were detected in more than 80% of the influent samples. Trichloromethane had the highest concentrations among the most frequently (80-100%) detected PSs in the influents and effluents. The potential risks that may arise from WWTP effluents containing PSs were estimated by calculating the risk quotients (RQs). Upon the risk estimation conducted on the PSs in effluents, monitoring of the endrin, alpha-cypermethrin, thetacypermethrin, zeta-cypermethrin, quinoxyfen, bifenox, benzo-ghi-perylene, and DEHP is recommended for the WWTP effluents.