S.D. Codified Laws § 54-3-1.1

Unless a maximum interest rate or charge is specifically established elsewhere in the code, there is no maximum interest rate or charge, or usury rate restriction between or among persons, corporations, limited liability companies, estates, fiduciaries, associations, or any other entities if they establish the interest rate or charge by written agreement. A written agreement is a document in writing, whether in physical or electronic form, in which the parties have demonstrated their agreement to the terms and conditions of an extension of credit, including the rate of interest. A written agreement includes the contract created by § 54-11-9.