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In this study, we investigated the rate and pattern of expansion of a non-native eelgrass, Zostera japonica, in relation to the distribution of the native eelgrass Zostera marina in a coastal estuary of the northeastern Pacific Ocean. The distributions of the Zostera congeners were monitored between 1997 and 2014 in Yaquina Estuary on the central Oregon coast, USA, using digital classification of color infrared aerial photographs and ground surveys. Correction factors for seasonal variations in cover were obtained to normalise the annual photo survey results to a common date (mid-August). Major expansions in the distributions of Z. japonica meadows over most of the 17-year study period were observed. However, there was no indication that the large (similar to 1500%) increase in areal extent of Z. japonica in the lower estuary between 1997 and 2007 was accompanied by a change in areal extent of the native Z. marina in this system.