Sea ducks have been captured with mist nets set across breeding streams and in drive traps at sea during the flightless period, but capture of flying birds on staging and wintering coastal areas presents a challenge. Here, we describe a highly successful technique for capturing Harlequin Ducks (Histrionicus histrionicus) at sea, modified from a mistnet set developed to capture Marbled Murrelets (Brachyramphus marmoratus) at sea. While the original method was passive, decoys were added to attract birds and occasionally birds were driven toward the nets using small boats. The capture technique proved to be safe and effective. Three hundred and seventy-eight birds were captured during 28 d of effort and no birds died in the nets or during handling. The technique has been further modified to capture molting Harlequin Ducks. The technique and its modified version could be easily used to capture other sea ducks on their staging, wintering and molting areas.