Land use and land cover changes (LULC) result in alterations to landscape structure, with particularly significant consequences in the landscapes of coastal basins due to their unique characteristics and special sensitivity. The aim of this work was to introduce a new methodology to assess the impacts of LULC transitions on landscape structure in a coastal basin of the Los Rios Region in Chile. Changes in landscape patterns were assessed by analyzing systematic transitions in conjunction with moving windows landscape metrics and spatial cluster analysis. An index measuring the impact of transitions on landscape structure change (ITSC) was calculated to assess the degree to which each systematic transition contributed to the spatial cluster of landscape change. The proposed method showed that transitions resulting from the replacement of native forest and especially those which involve its transformation into forestry plantations, have the greatest potential impact on landscape structure in the basin. Therefore, planning and management measures must be established to prevent such transitions, so avoiding a massive change in landscape structure.