S.D. Codified Laws § 29A-2-514

A contract to make a will or devise, or not to revoke a will or devise, or to die intestate, if executed on or after July 1, 1995, may be established only by (i) provisions of a will stating material provisions of the contract, (ii) an express reference in a will to a contract and extrinsic evidence proving the terms of the contract, or (iii) a writing signed by the decedent evidencing the contract. The execution of a joint will or mutual wills does not create a presumption of a contract not to revoke the will or wills.