| State: | SD |
| Type: | US |
| Route: | 16A |
| Section: | 139 |
| Map Custom: | yes |
| Map Notes: | US 16A highlighted in red |
| Length Mi: | 36.971 |
| Length Ref: | [1] |
| Direction A: | West |
| Terminus A: | in Custer |
| Direction B: | East |
| Terminus B: | near Keystone |
| Counties: | Custer, Pennington |
| Spur Type: | US |
| Spur Of: | 16 |
| Previous Type: | US |
| Previous Route: | 16 |
| Next Type: | SD |
| Next Route: | 17 |
U.S. Highway 16A (US 16A) is a 36.971adj=midNaNadj=mid scenic United States Numbered Highway. It is an alternate route for US 16. It splits from US 16 in the Black Hills of the southwestern part of the U.S. state of South Dakota. The highway's western terminus is an intersection with US 16, US 385, and South Dakota Highway 89 (SD 89) in Custer. The eastern terminus is at an interchange with US 16 called Keystone Wye south of Rapid City. Portions of US 16A are known as the Iron Mountain Road, named after the peak it summits.
left|thumb|Mount Rushmore seen from a tunnel on US 16AThe route passes through Keystone, the Mount Rushmore National Memorial, the Norbeck Wildlife Management Area and Black Elk Wilderness within Black Hills National Forest, and Custer State Park (including State Game Lodge and Legion Lake), before rejoining the parent highway.
US 16A is famous for its scenic, one-lane tunnels aligned to frame the faces on Mount Rushmore, its "pigtail bridges", and its sections of divided highway but with single (and narrow) lanes on each roadway. It is the only route which can be used to drive through Custer State Park without having to pay an entrance fee for the park, provided the traveler does not stop in the park.
The route includes most of the tunnels on the South Dakota state highway system, including the only three-lane tunnel in the state, just north of Keystone. Part of the highway is also a boundary of the Black Elk Wilderness. The Iron Mountain portion of the road is not maintained in the winter. The road, like several other scenic roads in the Black Hills, was originally laid out by Governor Peter Norbeck, specifically to create a very scenic, slow-speed road for tourists. The section of US 16A from SD 89 to SD 244 is known as the Peter Norbeck Memorial Byway in honor of the governor. At the highest point of the byway, on the summit of Iron Mountain, there is a small memorial to Governor Norbeck.
An older road through the Badlands of South Dakota (currently designated as SD 240) was designated US 16A between 1944 and 1980.