The Au Sable River is a 138-mile-long (222 km) river in the Lower Peninsula of the U.S. state of Michigan. Rising in the Northern Lower Peninsula, the river flows in a generally southeasterly direction to its mouth at Lake Huron at the communities of Au Sable and Oscoda.It is considered one of the best brown trout fisheries east of the Rockies and has been designated a blue ribbon trout stream by the Michigan Department of Natural Resources. A map from 1795 located in the United States Gazetteer calls it the Beauais River. In French, the river is called the Rivière au sable, literally "Sand River".

The Lower Peninsula of Michigan – also known as Lower Michigan – is the larger, southern and less elevated of the two major landmasses that make up the U.S. state of Michigan; the other being the Upper Peninsula, which is separated by the Straits of Mackinac. It is surrounded by water on all sides except its southern border, which it shares with Indiana and Ohio. Although the Upper Peninsula is commonly referred to as "the U.P.", it is uncommon for the Lower Peninsula to be called "the L.P."