Interstate 35W is the name of the western half of Interstate 35, which is used in two regions along its length. I-35 is a long north-south Interstate Highway having termini in Laredo, Texas and Duluth, Minnesota, and is the only existing Interstate highway to be 'split' into two separate interstates designated by location to serve different destinations. The reason for this is partly political, as both Minnesota and Texas have "twin cities" that each wanted Interstate 35 to themselves. Any alternate numbering scheme would likely cause "gaps" in the interstate, leading to confusion. The companion route to I-35W is the eastern Interstate 35E.

Interstate 35W in Minnesota

Interstate 35 splits in eastern Minnesota at Forest Lake in the north and Burnsville to the south. I-35W runs through Minnesota's largest city, Minneapolis, while I-35E serves neighboring St. Paul, the state capital. I-35W essentially supplants the now-decommissioned highways of old U.S. 8 north of Minneapolis, and old U.S. 65 south of the city.

Mileposts and exits are numbered independently on I-35W, while they are contiguous with the rest of the interstate on I-35E. Exit number 1 on I-35W is in Forest Lake. The highway itself is marked as an "exit" from I-35E. This segment is 39 miles in length, about two miles shorter than its companion route.

Interstate 35W in Texas

Interstate 35 splits in northern Texas at Denton to the north and Hillsboro to the south. I-35W passes through Fort Worth, while the more easterly Interstate 35E passes through Dallas. I-35W supplants the old U.S. Highway 81 through and south of Fort Worth and largely supplants the extant U.S. Highway 377 between Denton and Fort Worth. Denton, Fort Worth, and Waco are control cities on this highway. Interstate 35W is the more direct route for long-distance north-south expressway traffic, as is noted on signs north and south of the 35W/35E splits near Denton and Hillsboro.

I-35W is 85 miles in length, nearly 12 miles shorter than its eastern companion. The milepost numbering arrangement is just like what is done in Minnesota, although the lowest-numbered exit is at the southern end of the highway.