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Satellite observations and modeling data have suggested a significant increase in net primary production in the Arctic Ocean over the last decade due to retreating sea ice and the development of light availability caused by Arctic warming. Subsequently, under‐ice blooms (UIBs) are being recognized as an important phenomenon from the traditional perspective. However, the role of sea‐ice algae in UIBs is still unknown due to the limited availability of continuous observations. We analyzed data on primary producer‐derived lipid biomarkers from sinking particles collected over 1 year using time‐series sediment traps on the East Siberian Sea and Chukchi Sea slopes. Based on the seasonal changes in sympagic organic carbon derived from the data of the ice proxy (IP25) flux and pelagic biomarkers, such as highly branched isoprenoid trienes, epi‐brassicasterol and dinosterol, a UIB was identified in summer 2018 on the East Siberian Sea slope. Compared to the nutrient distribution on the Chukchi Sea slope, the UIB on the East Siberian Sea slope might have been triggered by the nutrient supply. The estimated flux‐weighted mean sympagic organic carbon value measured during the UIB period (May−August) was 1.04 mg m−2 d−1 on the East Siberian Sea slope, approximately five times greater than recorded that on the Chukchi Sea slope (0.23 mg m−2 d−1) during the same period. Our findings suggest that the importance of sea‐ice algae as primary producers has increased as the UIB phenomenon has become more important in the Arctic Ocean and that sea‐ice environments face changes due to Arctic warming. Plain Language Summary: The production of phytoplankton in the Arctic Ocean is dramatically increasing due to sea‐ice losses caused by Arctic warming. This increase in production can also be observed in the production of phytoplankton living under sea ice. We determined the origins of these phytoplanktons through a chemical analysis of particles falling from the sea surface to the sea bottom in the Arctic Ocean. Considerable under‐ice phytoplanktonic growth caused by the nutrient supply was identified on the East Siberian Sea slope. In particular, the production of under‐ice phytoplankton on the East Siberian Sea slope was confirmed to increase by approximately five times compared to that measured on the Chukchi Sea slope during the high‐growth period (May−August). Our study suggests that under‐ice phytoplankton may play an increasingly important role as primary producers in the Arctic Ocean, a region in which sea‐ice environments face changes due to Arctic warming. Key Points: Under‐ice blooms may be enhanced by the development of light availability and triggered by the nutrient supplyThe flux of ice‐related organic carbon during under‐ice blooms increased five times compared to that measured in nonbloom conditionsLipid biomarker analysis approach using sinking particles could be an important solution for understanding the under‐ice blooms [ABSTRACT FROM AUTHOR]