冲积平原，又称泛滥平原，是由河流沉积作用在中游冲积扇及河口三角洲间形成的下游平原地貌。在河流的下游，由于水流没有上游般急速，而下游的地势一般都比较平坦。河流从上游侵蚀了大量泥沙，到了下游后因流速不再足以携带泥沙，结果这些泥沙便沉积在下游。尤其当河流发生水浸时，泥沙在河的两岸沉积，冲积平原便逐渐形成。

基本上任何河流在下游都会有沉积现象，尤其是一些较长的河流为甚。但也有部分大河没有形成下游冲积平原及河口三角洲，如怒江、刚果河、圣弗朗西斯科河等。世界上最大的冲积平原是亚马逊平原，乃由亚马逊上游的泥沙堆积而成。而中国的黄淮海平原和长江中下游平原亦属这一地形。

An alluvial plain is a largely flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms. A floodplain is part of the process, being the smaller area over which the rivers flood at a particular period of time, whereas the alluvial plain is the larger area representing the region over which the floodplains have shifted over geological time.