A new hybrid microwave (MW) and ultrasound (US)-assisted reactor concept was investigated. The 2.45 GHz microwaves were delivered by a semiconductor generator via a coaxial cable to a monomode Transverse Electric (TE) microwave resonant cavity within which the reactor was placed; the US (25 kHz) were delivered at the bottom of the TE cavity via a novel designed sonotrode consisting of a detachable metallic plate-probe (indirect sonication). The semiconductor microwave generator helped to optimize the absorbed energy via its automatic frequency tuning function. The dual MW/US device allowed the use of both technologies separately or in a simultaneous combined manner. The ring-closing metathesis of diethyl diallyl malonate in glycerol micellar conditions was studied as an example using this novel hybrid technology. The results were compared with those obtained when microwaves or ultrasound were applied individually. Various benzylidene-, indenylidene- and Hoveyda–Grubbs-type catalysts were screened. The novel reactor for combined MW/US irradiation showed beneficial effects on the outcome of the reaction.