Abstract We herein report the first use of thiosquaramides as polymerization catalysts, which are shown to be effective for the controlled ROP of lactide in the presence of an alcohol source and NEt 3 . Comparison of their catalytic performances with the less acidic squaramides are also discussed. The observed catalytic activity of variously N ‐substituted thiosquaramides suggest that a balanced NH Brønsted acidity is required for optimal performance. Most interestingly and rather unexpectedly, DFT‐supported calculations on thiosquaramide‐mediated lactide ROP catalysis suggest that secondary interactions between the thiosquaramide N ‐substituents and the incoming lactide (presently of type C‐H⋅⋅⋅ π ‐arene) are crucial for catalytic activity. Though this type of interactions is quite common in organo‐catalysis, it has rarely been evidenced to play a key role in the area of organo‐catalyzed polymerizations. Such catalyst substituents/substrate interactions may well play a significant role in the catalytic performances of various other systems.