Muonium (Mu = µ+e–) is composed of a muon of light isotope of proton (µ+) and electron (e–) and can be used as a light surrogate for a hydrogen atom. In this paper, we investigated addition of muonium to a newly synthesized Mes*-substituted thioformamide (Mes*NHCH=S, Mes* = 2,4,6-tBu3C6H2). Transverse-field muon spin rotation (TF-µSR) of a solution sample of the thioformamide confirmed addition of muonium to the sulfur atom leading to the corresponding C-centered radical [Mes*NHC(H)·-SMu]. Density functional theory (DFT) calculations assigned a conventional amino(mercapto)methyl radical, in which both nitrogen and carbon were slightly pyramidalized, and the calculated muon hyperfine coupling constant (hfcc) including the muon isotope effect was compatible with the experimentally determined parameter. However, the muon level-crossing resonance (µLCR) spectrum of an anisotropic crystalline sample indicated two paramagnetic species, and the major product showed the considerably larger muon hfcc compared with the conventional structure of the amino(mercapto)methyl radical. The unusual transient muoniated thioformamide with the larger muon hfcc that showed rapid relaxation could be only explained by a transient structure including planarization of the nitrogen and carbon atoms in Mes*NHC(H)·-SMu.