Abstract Incised valleys associated with sequence boundaries of regional extent are present at two stratigraphic levels in Neoproterozoic siliciclastic rocks of northern and western Utah and southeastern Idaho. In comparison with many Phanerozoic examples, the sedimentary fill of these Neoproterozoic incised valleys is unusually coarse-grained. The most prominent paleovalley system is present along a sequence boundary in the upper part of the Caddy Canyon Quartzite, and may be traced from the Portneuf Range in southeastern Idaho south to the Canyon Range in central Utah and west to the Dugway Range in western Utah. Individual valleys range in depth from several meters to >45 m and in width from a few tens of meters to several hundred meters. Valley fills consist of diffusely to well stratified granule to pebble conglomerate characteristically containing siltstone clasts < cm to >2 m across. They are interpreted to have accumulated predominantly in a fluvial environment, and perhaps in part by debris flows in a subaerial or shallow estuarine setting. This paleodrainage system is of significance because it is one of the few documented examples of a widespread incised-valley system that erodes into an underlying fluvial braid plain more than 200 km in width, indicating that the effects of base-level changes were felt far upstream in the more proximal reaches of a braided fluvial system. A second incised valley with more than 60 m of local relief, and located at the sequence boundary at the base of the Geertsen Canyon Quartzite, is developed only locally in the Portneuf Range of southeastern Idaho. It is similar in both geometry and sedimentary fill to the valleys in the Caddy Canyon Quartzite, and is incised into the proximal reaches of a widespread fluvial braid plain. Somewhat shallower, conglomerate-filled valleys associated with higher-order cyclicity have been observed within the upper part of the Caddy Canyon Quartzite and overlying Inkom Formation, and, unlike the other examples, are encased in offshore marine siltstone. The development of Neoproterozoic sequence boundaries in the western U. S. is probably related to some combination of glacial eustasy (well documented in correlative strata elsewhere) and lithospheric extension, which appears to have preceded the development of the early Paleozoic passive continental margin. The relative roles of these mechanisms cannot yet be distinguished in the absence of more precise geochronology.