| State: | SD |
| Type: | SD |
| Route: | 42 |
| Map Notes: | Route of SD 42 (in red) |
| Section: | 162 |
| Length Mi: | 71.973 |
| Length Ref: | [1] |
| Direction A: | West |
| Direction B: | East |
| Section1: | Western segment |
| Length Mi1: | 5.870 |
| Terminus A1: | in Aurora County |
| Terminus B1: | Aurora–Davison county line |
| Section2: | Central segment |
| Length Mi2: | 59.231 |
| Terminus A2: | west of Ethan |
| Junction2: | in McCook County |
| Terminus B2: | Sioux Falls |
| Section3: | Eastern segment |
| Length Mi3: | 6.872 |
| Terminus A3: | Six Mile Road in Sioux Falls |
| Terminus B3: | west of Benclare |
| Previous Type: | SD |
| Previous Route: | 41 |
| Next Type: | SD |
| Next Route: | 43 |
South Dakota Highway 42 (SD 42) is a segmented state highway in southeastern South Dakota, United States. The first segment is a 6miles highway in Aurora County. The second and longest is 60miles long and connects Ethan and Sioux Falls. The final segment connects Sioux Falls to Iowa Highway 9 southeast of Rowena.
The westernmost segment of SD 42 begins at a junction with U.S. Route 281 (US 281) and travels east through open farmland. This is the shortest of the three segments; it is only about 6miles long, and it ends as the roadway crosses from Aurora County into Davison County.
The central segment of the highway begins at an intersection with SD 37 in rural Davison County. It heads east and crosses the BNSF Railway.[2] Just east of the railroad, the route passes through the town of Ethan, then enters Hanson County. About 6miles farther east, SD 42 bends slightly to the south and crosses the James River. The highway continues east through the flat prairie and enters McCook County.
About 5miles east of the county line, the highway enters the city of Bridgewater. Here, it curves to the northeast and crosses another branch of the BNSF Railway[2] before meeting the eastern terminus of SD 262. SD 42 then curves back to the east and leaves Bridgewater. It continues through flat farmland, intersecting US 81 and passing over the west fork of the Vermillion River. The route heads east about 10miles before crossing the east fork of the Vermillion River, and then entering Minnehaha County.
In Minnehaha County, SD 42 intersects SD 19, and the two highways run east for about 1miles before SD 19 splits off to the north, and SD 42 continues east toward Sioux Falls. It intersects the northern terminus of SD 17 approximately 3miles west of the city. The central segment of SD 42 ends at Ellis Road near the Sioux Falls city limits.
The easternmost segment of SD 42 begins at Six Mile Road east of the city of Sioux Falls. It runs southeast, concurrent with SD 11 along Arrowhead Parkway. The two highways pass by a golf course and a park before crossing the Big Sioux River. East of the river, SD 11 turns to the north toward Brandon, and SD 42 curves to the east. The highway passes through the unincorporated community of Rowena before curving due south and crossing the Iowa state line. The route continues into Iowa as Iowa Highway 9.