The role of water in the methanol-to-olefins (MTO) process over H-SAPO-34 has been elucidated by a combined theoretical and experimental approach, encompassing advanced molecular dynamics simulations and in situ microspectroscopy. First-principles calculations at the molecular level point out that water competes with methanol and propene for direct access to the Bronsted acid sites. This results in less efficient activation of these molecules, which are crucial for the formation of the hydrocarbon pool. Furthermore, lower intrinsic methanol reactivity toward methoxide formation has been observed. These observations are in line with a longer induction period observed from in situ UV-vis microspectroscopy experiments. These experiments revealed a slower and more homogeneous discoloration of H-SAPO-34, while in situ confocal fluorescence microscopy confirmed the more homogeneous distribution and larger amount of MTO intermediates when cofeeding water. As such, it is shown that water induces a more efficient use of the H-SAPO-34 catalyst crystals at the microscopic level. The combined experimental-theoretical approach gives a profound insight into the role of water in the catalytic process at the molecular and single-particle level.