^Interstate 39 starts off toward the south in Wausau, July 2015.

In Wisconsin, Interstate 39 runs north-south in central and south-central Wisconsin, from Wausau to Beloit. Speaking more broadly, Interstate 39 continues south into north-central and central Illinois, but in Wisconsin, it follows the lower half of Wisconsin's backbone. Interstate 39 enters Wisconsin by crossing the Illinois-Wisconsin state line near the exact middle. Interstate 90 joins Interstate 39 northward by Rockford, and the two routes enter Wisconsin together at Beloit. Interstates 39 and 90 meet the southwestern start point of Interstate 43 in Beloit before they continue northward together to Janesville and Madison. On the east side of the Madison area, Interstate 94 comes from the west to join Interstates 39 and 90 northward. The now triple-numbered Interstate freeway continues northward to Portage, where Interstates 90 and 94 turn westward to the Wisconsin Dells. This leaves Interstate 39 heading northward on its own past Portage. US Route 51 joins on the north side of Portage, and US Route 51 and Interstate 39 continue northward through central Wisconsin together. They pass by Stevens Point, and then they approach Wausau. In Wausau, Interstate 39 ends at the east-west freeway carrying Wisconsin Highway 29, but the northward freeway continues beyond this point, carrying US Route 51.

My photo of a route marker for Interstate 39 in Wisconsin shows a pair of junction markers found along Wisconsin Highway 81 in Beloit. Wisconsin Highway 81 crosses Interstates 90 and 39 at the same place where Interstate 43 begins toward the northeast; that flyover ramp in the foreground is a ramp leading to the northeastward beginning of Interstate 43. Those looking west toward the interchange at Highway 81 and Interstate 90 are introduced to the situation by these junction markers. This photo was taken in Beloit on my way north to northwestern portions of Wisconsin and eastern portions of Minnesota in July 2025.
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