Plastic accumulation is one of the main environmental issues of our time. In 2016, two enzymes capable of degrading polyethylene terephthalate (PET), one of the most common plastic polymers, were discovered. PETase and MHETase from Ideonella sakaiensis (IsPETase and IsMHETase, respectively) work sequentially to degrade PET to its constituent monomers. PETase catalyzes the cleavage of PET repetitive units ((mono-(2-hydroxyethyl)terephthalic acid (MHET)), whereas MHETase hydrolyses MHET into terephthalic acid (TPA) and ethylene glycol (EG). In this work, the catalytic mechanism of IsPETase was studied by QM/MM. The reaction was found to progress in four distinct steps, divided into two major events: formation of the first transition intermediate and hydrolysis of the adduct. The transition state and respective reactant and product of each step were fully characterized and described. The rate-limiting step was found to be step 3, with an activation barrier of 12.5 kcal mol⁻¹. Furthermore, in this study, we have shown the critical role of a triad of residues composed by Ser207, Ile208, and Ala209 in stabilizing the catalytic Asp206 residue. This finding confirms the importance of using a larger QM region since our results disclose some important differences when compared with previous computational studies of the same mechanism. These results provide valuable insights into the catalytic mechanism of IsPETase that can contribute to the rational development of more efficient engineered enzymes.