Quantum tunneling is computed for a reaction sequence that models the conversion of the ortho-quinone methide of cannabigerolic acid 1 to the decarboxylated product (−)-trans-Δ9-tetrahydrocannabinol (THC, 3). This calculation is the first to evaluate multidimensional tunneling in this sequence. Computations were carried out with POLYRATE and GAUSSRATE using B3LYP/6-31G­(d,p) to examine the mechanism of THC 3 formation. The pentyl chain on THC 3 and its precursors were replaced with a methyl group to compute tunneling contributions to the rates of four separate steps: (i) initial Diels–Alder reaction of the quinone methide with the trisubstituted alkene end-group of the geranyl 1Z-CH 3 to give 2Z-CH 3 , (ii) acid-catalyzed keto–enol tautomerization, which converts 2rZ-CH 3 to 4rZ-CH 3 , (iii) carboxyl rotamerization converting 4rZ-CH 3 to 4E-CH 3 , and (iv) decarboxylation that converts 4E-CH 3 to 3-CH 3 . Tunneling contributions to the rate constants of steps (i)-(iv) are between 19 and 76% at 293 K. In step (ii), nonuniform changes in the zero-point vibrational energy along the reaction path created a shallow minimum in the 0 K free energy. It is a hidden intermediate because it is not a minimum on the potential energy surface and is detectable only when zero-point energy is taken into account along the reaction path. Predicted kinetic isotope effects would be experimentally observable at temperatures that are convenient to use. This is particularly relevant in the decarboxylation stage of the reaction sequence and has important implications because of its role in THC 3 formation.