The Northwestern Union Railway was a railroad company in the United States.

It was incorporated in 1871 and opened a line between Milwaukee and Fond du Lac, Wisconsin, in 1873.

[1] The company's 62.63-mile (100.79 km) line between Milwaukee and Fond du Lac, Wisconsin, opened in 1873.

At the northern end in Fond du Lac, it connected with the Sheboygan and Fond du Lac Railroad's line between Sheboygan and Princeton, and the Chicago and North Western Railway's line from Chicago to Green Bay via Janesville.

During the era of Chicago and North Western passenger service it hosted Upper Peninsula-bound trains such as the Flambeau 400 and Peninsula 400.