Least Bittern (Ixobrychus exilis) populations have declined in Canada, and this has led to the species being listed as threatened under the Species-at-risk Act. Wetland loss and degradation has been extreme in southern Ontario (> 90% loss in some areas) and this loss has been identified as a potential contributor to population declines. The objective of this study was to determine the influence of land-use surrounding wetlands, wetland size, isolation and water quality in predicting the presence of the Least Bittern in southern Ontario coastal marshes. Between 2006 and 2008, we surveyed 20 coastal marshes for Least Bitterns in southern Ontario. The proportion of urbanized land within 4,000m of the marsh was the most important variable in predicting their presence/absence. Least Bitterns were more likely to be found in wetlands surrounded by low levels of urbanization and high levels of rural land-uses. We also found that sites with higher levels of urbanization and poorer water quality, as indicated by the Water Quality Index (WQI), were less likely to support Least Bitterns. Water quality may impact the ability of this species to forage successfully and may be one potential route through which the Least Bittern is impacted by urbanization. Based on these results, urbanization should be limited within 4,000m of the wetland edge to protect existing rural marshes from degradation. Further research is needed to identify the impact that small-scale patterns of urbanization may have on habitat of Least Bitterns in Great Lakes coastal marshes.