SDCL § 7-1-61: Todd County boundaries.

Beginning at the intersection of the third guide meridian west of the sixth principal meridian with the boundary line between the states of South Dakota and Nebraska; thence north and along said third guide meridian to the township line between townships thirty-nine and forty, north; thence west and along said township line to its intersection with the western boundary line of the Rosebud Indian Reservation; thence south and along said boundary line of said reservation to the boundary line between the states of South Dakota and Nebraska; thence east and along said state line to the place of beginning.