Abstract— <p>The first data on the breeding biology of the Pacific eider (<i>Somateria mollissima v-nigrum</i>) in Asia have been obtained using the example of the nesting population in the Apapelgin River (Western Chukotka). From 2018 to 2024, research was conducted to assess breeding density, phenology, and reproductive parameters of the eider. A total of 1291 nests of this species have been examined. This study presents data on the key parameters of the investigated breeding population: eiders predominantly nested in coastal lowlands rich in lakes, known as “laida.” The average breeding density was 26.8 ± 2.3 nests/km², showing a declining trend over the years (<i>R</i>² = 0.7). The average clutch size was 4.7 ± 0.1 eggs. Clutch onset was influenced by the rate of snowmelt and the average temperatures during the last decade of May. The apparent nest success across all years was 33.4%, and the cumulative Mayfield nest success (MNS) estimate during the egg-laying and incubation periods was 0.37 ± 0.14. The egg volume in clutches decreased over the study period and was influenced by weather conditions at the beginning of the breeding season, the date of egg laying, and clutch size. The data are analyzed in comparison with similar data on nesting populations from the continent of America.</p>