U.S. Route 12 in Indiana

Burns Harbor section near intersection with SR 149

US 12 enters Indiana concurrent with US 20 and US 41, at which point it passes under the Indiana Toll Road. The road passes both commercial and industrial areas between Wolf Lake and Horseshoe Hammond. US 12, US 20, and US 41 are concurrent until US 41 turns south on Calumet Avenue. Both US 12 and US 20 head southeast toward East Chicago, where US 12 turns east while US 20 continues south. The route heads east through East Chicago as a four-lane undivided highway passing through mostly residential areas. The road heads south concurrent with SR 912, passing through mainly industrial area between East Chicago and Gary. The two routes have an interchange at SR 312 and access to the Indiana Toll Road, via an interchange at Gary Avenue. The road has two bridges with the first passes over the Grand Calumet River and the other passing over the Indiana Toll Road and the South Shore Line.[10][11][12]

After passing the rail line, the road has an interchange with US 20, this interchange is the southern end of the SR 912 concurrency. US 12 follows US 20 east toward Downtown Gary, as a four-lane divided highway. The two highways pass through residential areas and crosses the Lake Shore and Michigan Southern Railway owned by Norfolk Southern Railway, before Downtown Gary. At Bridge Street in Gary, the road becomes one-way streets, with westbound on Fourth Avenue and eastbound on Fifth Avenue. In Downtown Gary it has an intersection with SR 53, locally known as Broadway. The streets also pass by the historical Knights of Columbus Building and the old Ralph Waldo Emerson School building. East of Downtown Gary, the road passes by U.S. Steel Yard, the home of the Gary SouthShore RailCats, an independent professional baseball team. The one-way streets end and the road becomes a six-lane divided highway, known as Dunes Highway. The Dunes Highway passes between the South Shore commuter rail line and residential houses. East of Gary the route has a signalized intersection with the northern terminus of I-65.[10][11]

US 12 (Dunes Highway), westbound in the Dune Park section

After I-65, the road has a trumpet interchange with I-90. In the far eastern portion of Gary, US 12 and US 20 split for the final time, although US 20 closely parallels US 12 for the next 20 miles (32 km), sometimes coming within 0.2 miles (0.32 km). US 20 follows a slightly more southerly route to Michigan City (via Portage and Porter), while US 12 passes through Indiana Dunes National Park and retains the name "Dunes Highway". After splitting from US 20, US 12 becomes a mostly rural two-lane highway, passing through the woodland.[11][13]

The road passes just south of the historical Miller Town Hall, in the community of Miller Beach. After the community of Miller Beach, the highway enters Indiana Dunes National Park. The road passes through woodland and parallel to the South Shore tracks, before leaving the national park west of Ogden Dunes. In Ogden Dunes, the highway becomes a four-lane divided highway, passing through woodland with some houses. After Ogden Dunes, the road has a folded diamond interchange with the north terminus of SR 249 and George Nalson Drive, which heads into the industrial complex at Burns Harbor. The industrial complex at Burns Harbor includes a NIPSCO powerplant and a steel mill. After SR 249, the route has an intersection with SR 149 and a folded diamond interchange at an access road into Burns Harbor. After this interchange, the highway narrows to a two-lane highway again.[11][13]

The highway is near, but not on or within sight of, the shoreline of Lake Michigan. The route reenters Indiana Dunes National Park at a traffic light for the eastern entrance of Mittal Steel Company. After the traffic light the road passes south of Dune Acres and enters Indiana Dunes State Park. The road has an intersection with SR 49, with access to the lake front. After the intersection, the road passes under the access road to the lake front. The highway leaves the dunes and enters Beverly Shores. In Beverly Shores at the intersection of US 12 and Broadway is Beverly Shores station. After leaving Beverly Shores, the route enters Town of Pines and has an all-way stop at the northern terminus of SR 520. SR 520 is the final connecting route to US 20 before the two separate regionally.[11][13]

After Town of Pines, the road curves due north back into the dunes and crossing the South Shore Line tracks. The route curves northeast and becomes a four-lane undivided highway, before leaving the dunes. After the dunes the route crosses the Michigan Line, owned by Amtrak. The Michigan Line is used by Amtrak's Wolverine train. The route makes a few more curves before entering downtown Michigan City and passing through a mix of industrial and commercial areas. In downtown Michigan City, the route becomes a four-lane divided highway and passes south of the Old Michigan City Light but just north of the Lighthouse Place Premium Outlets. Before exiting Michigan City, the road narrows to a four-lane undivided highway and intersects SR 212, an eastern bypass of Michigan City. After SR 212 the route leaves Michigan City and passes through Long Beach. The highway leaves Long Beach, traveling through woodlands with houses and the town of Michiana Shores, before entering Michigan.[11][13]