S.D. Codified Laws § 25-6-18

An adult may adopt another adult by filing a petition requesting the adoption with the judge of the circuit court, together with an agreement in writing that the person being adopted shall be treated in all respects as a natural child of the petitioner. Written consent of the adopted person shall also be required. It shall be a further prerequisite that the person being adopted shall have lived in the home of the adoptive parent for a period of at least six months while the person being adopted was under twenty-one years of age, and this fact shall appear in the petition. If the person being adopted is the biological child of the adoptive parent, the prerequisite of living in the home of the adoptive parent during the person's minority is waived.