We examined the biogeography of aquatic and semiaquatic Heteroptera (ASH) in the Grand Canyon (GC) ecoregion (GCE) on and adjacent to the southern Colorado Plateau. We report 89 ASH taxa in 86 species, 37 genera, and 14 families in the GCE, including 54 ASH taxa detected within or on the rims of GC and its major tributaries, a fauna 3.8-fold greater than previously reported. We tested 2 groups of biogeographic hypotheses to account for this high level of diversity, demonstrating an underlying pattern of mixed biogeographic affinity and strong landform-climate effects. Equal numbers of ASH taxa were derived from allochthonous (neotropical and nearctic) sources and autochthonous (range-centered) sources. A negative linear relationship existed between area-adjusted ASH taxon density and elevation, with more Mexican/neotropical taxa at low elevations and more nearctic taxa at higher elevations. While species richness was positively scale dependent, biogeographic landform impacts were unrelated or negatively related to spatial scale. The uplifted southern margin of the Colorado Plateau along the Mogollon Rim supported elevated ASH diversity as a function of ecotone effects and interprovincial basin connectivity. Barrier/filter effects were stronger than null, or refuge effects, and little endemism was detected in the GCE. Colonization history varied across elevation and in relation to landscape evolution. No reported GCE taxa have been extirpated, but 52.8% of the fauna occurred at 3 or fewer localities (primarily springs), sites that may be threatened by habitat alteration and climate change.