We report the diastereoselective and enantioselective synthesis of thio-substituted cyclobutanes via a sulfa-Michael addition using cyclobutenes. In the presence of DBU, various thio-cyclobutane esters and amides were obtained in up to quantitative yield and >95 : 5 dr. Using a chiral chinchona-based squaramide bifunctional acid-base catalyst and an N-acyl-oxazolidinone-substituted cyclobutene, thio-cyclobutanes were obtained with high yield and enantioselectivity (er up to 99.7 : 0.3).