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Providing passage at man‐made barriers is important to preserving migratory fish species. Fishways used to bypass barriers have historically been designed to provide passage for economically important species, with little regard given to nontarget species. Consequently, native fish species whose swimming behaviors are not well suited to these fishways have suffered from restricted access to habitat. For example, poor passage at dams has contributed to declines in populations of Pacific Lamprey Entosphenus tridentatus in the Columbia River basin. Pool‐and‐weir fishways typical of Columbia basin dams are well suited to salmonids but not to anguilliforms like the Pacific Lamprey. We evaluated the passage efficiency of Pacific Lampreys through a new pool‐and‐weir fishway at River Mill Dam on the Clackamas River, Oregon, that was designed to facilitate Pacific Lamprey passage. Rounded corners at the fishway entrances, flush‐mounted weir gates, chamfered corners on orifices and weir walls, and orifices flush with the floor were all included in the fishway design specifically for Pacific Lampreys. In 2013 and 2015, Pacific Lampreys were radio‐tagged and PIT‐tagged to assess passage success. Dam passage efficiency estimates ranged from 84% to 98%, roughly 10–50% higher than Pacific Lamprey passage efficiency estimates at other dams in the Pacific Northwest. The median passage time through the fishway was 0.87 d in 2013 and 0.71 d in 2015. The high passage efficiency of Pacific Lampreys at River Mill Dam is likely due to the design elements incorporated specifically for Pacific Lampreys. These features could inform future fishway designs and modifications at locations where Pacific Lamprey passage is a consideration. [ABSTRACT FROM AUTHOR]