U.S. state flag consisting of a blue field (background) with the state seal in the centre encircled with the name of the state and the inscription “The Mount Rushmore State.”

In the early 20th century the secretary of the state historical society, Doane Robinson, worked with the state senator Ernest May to develop a flag for South Dakota, the design for which was first painted by Robinson's secretary, Ida Anding. The flag, influenced by a song written by Willis Johnson, had a sky-blue field featuring a yellow sun with rays, framed by the name of the state and its nickname “The Sunshine State.” After the legislature added the state seal on the reverse side, the new flag became official July 1, 1909. The expense and practical difficulties of making a double-sided flag eventually convinced state authorities to amalgamate the two central emblems.

On March 11, 1963, the state legislature approved a new design for use on both sides of the flag. The inscriptions remained, but the South Dakota seal was represented on top of the sun in such a way that only the sun's rays were visible. Another modification was introduced on July 1, 1992, when the new state nickname—“The Mount Rushmore State”—was substituted for the old wording. The seal repeats the name of the state, the date of its admission to the Union (1889), and the state motto, “Under God the people rule.” A farmer is shown in the foreground plowing his fields. In the background cattle, crops, a smelting furnace, and a steamship on a river complete the naturalistic details of the design.