In summary, we describe the cooperative activation of molecular H2 by a nickel-olefin complex. Incorporating a precoordinated olefin in a pincer ligand framework allowed for the direct observation of a rapid chemical exchange between free H2, a non-classical (olefin)Ni-(H2) complex, and an alkyl(hydrido) nickel(II) species by NMR spectroscopy. Experimental observations and DFT calculations support a Ligand-to-Ligand Hydrogen Transfer (LLHT) mechanism, that is a concerted H atom transfer from the metal-coordinated H2 molecule to the olefin without prior oxidative addition of H2. The reported system is an active catalyst for the semihydrogenation of diphenylacetylene using molecular H2. Mechanistic investigations and DFT studies illustrate the importance of LLHT steps for nickel, which contrasts with heavier group 10 elements. These results provide an experimental basis for considering LLHT steps in olefin hydrogenation mechanisms, especially with catalysts based on first-row transition metals. Furthermore, they demonstrate the potential of a tethered C=C double bond in the cooperative activation of small molecules, opening a new avenue for the design of environment-friendly cooperative catalysts using non-noble metals. Further applications of this concept are currently investigated in our laboratory.