Despite their record-braking sorption capacities, metal−organic frameworks (MOFs) have rarely been used for the immobilization of homogeneous catalysts by simple absorption from solution. Here we demonstrate that this simple strategy allows the first successful immobilization of olefin metathesis catalysts inside MOFs. Ruthenium alkylidene complexes bearing ammonium-tagged NHC ligands were successfully supported inside (Al)MIL-101-NH2·HCl. The materials thus obtained are true heterogeneous catalysts, active toward various substrates with TONs up to 8900 (in batch conditions) or 4700 (in continuous flow). Although the catalysts were held inside the MOF by noncovalent forces only, leaching was not observed and heavy metal contamination of the products was found to be below the detection limit of ICP MS (0.02 ppm). The robustness of the catalyst attachment allowed their use in a continuous flow setup.