The ecological status of Lake Sevan, the largest freshwater reservoir in Armenia, is increasingly threatened by legacy agricultural pollution. This study assessed the transport of organochlorine pesticides from surrounding agricultural soils into Lake Sevan and its inflowing rivers via snowmelt and precipitation-driven runoff. Water sampling was conducted during November-December 2024 and January-March 2025. A total of 65 water samples were collected from seven nearshore zones of the lake and the mouths of six major inflowing rivers. Qualitative and quantitative analyses were performed using gas chromatography with an electron capture detector. This study presents the first seasonally resolved, basin-scale quantitative assessment of organochlorine pesticides in Lake Sevan and its major tributaries. DDT, DDD, DDE, heptachlor, α-HCH, and lindane (γ-HCH) were detected in surface waters. Concentrations increased after the snowmelt period, particularly in March 2025, and frequently exceeded threshold values. Concentration ratios of DDT/DDD and γ-HCH/α-HCH indicate historical pesticide use and ongoing remobilization from contaminated soils. The results show that surrounding agricultural lands act as active secondary sources of organochlorine pesticide contamination, posing risks to aquatic ecosystems and fish populations. Long-term monitoring and environmentally sound remediation strategies are required to protect Lake Sevan.