Massive Arctic rivers are feeding ≈11% of the global river discharge into the Arctic Ocean, while the ocean stores only ≈1% of the global ocean volume. The ongoing rapid climate warming has led to pronounced changes in precipitation, active layer thickening, increased air and soil temperatures, increased riverbank and coastal erosion rates, extensive permafrost thaw and increasing freshwater discharge to the Arctic Ocean. Since most studies have focused on rivers or oceans itself and mainly during the late summer, near-shore coastal regions are understudied and crucial in determining the amount of carbon transported and/or released into the Arctic Ocean. Here, we investigated river-derived carbon dioxide (CO2) and methane (CH4) emissions from seven repeated transects of the Kolyma River and nearshore (120 km between Cherskiy and Ambarchik) over the entire open water season between June and September 2019. We estimated the cumulative gross delivery of river-derived CH4 and CO2 to the coastal ocean to be around 0.0008 Tg CH4 (800 000 kg) and 0.2 Tg CO2 (200 000 000 kg). Measurements reveal that more than 50% of the cumulative gross delivery is happening during the fresh period, making the season dynamics extremely important.