An original concerted antarafacial mechanism for the addition of diorganosilyl‐zinc reagents across the C–C triple bond in ynamides is computationally investigated using DFT calculations. This concerted mechanism, leading to a trans‐product in only one step, results in the formation of a Si–C and a Zn–C σ‐bonds on opposite sides of the π‐system. We demonstrate that the mechanism going through a η²‐vinyl intermediate and the proposal of a radical chain pathway are energetically unsustainable. The retained concerted antarafacial pathway is tested on experimental selectivities: the regioselectivity, in favor of the silyl β‐addition in ynamide, and stereoselectivity, which is cis‐ with (Me₂PhSi)₂Zn but trans‐ with [(Me₃Si)₃Si]₂Zn, are well reproduced by DFT calculations. The regio‐ and stereoselectivity are discussed using the activation strain model and a chemical bonding analysis.