tert‐Butylthiophosphinidene complex [tBuSP‐W(CO)5] was generated by dissociation of 1‐(tert‐butylthio)phosphirane–W(CO)5 complex under mild conditions. The formation of transient [tBuSP‐W(CO)5] was indicated by trapping reactions with 2,3‐dimethyl‐1,3‐butadiene, alkynes, phenanthrene‐9,10‐dione, and methanol. The LUMO of [MeSP‐W(CO)5] is significantly lower in energy than those of [Me2NP‐W(CO)5], [MeOP‐W(CO)5], and [Me2PP‐W(CO)5]. The HOMO of [MeSP‐W(CO)5] contains a significant contribution from the in‐plane lone pair of P and the LUMO shows a typical π* characteristic. Since stabilized by sulfur lone pair and coordinated by W(CO)5, [tBuSP‐W(CO)5] undergoes facile and reversible cycloadditions with alkenes and alkynes.