S.D. Codified Laws § 12-4-5.4

Any person registering to vote shall provide the person's valid South Dakota driver license number or a South Dakota nondriver identification number on the voter registration form. If a person does not have a valid South Dakota driver license or a South Dakota nondriver identification number, the person shall provide the last four digits of the person's social security number on the voter registration form. If a person does not have a valid South Dakota driver license, a South Dakota nondriver identification number, or a social security number, the person may only register at the county auditor's office and shall sign a statement verifying the fact that the person does not have a valid South Dakota driver license, a South Dakota nondriver identification number, or a social security number. The statement shall be prescribed by the State Board of Elections.