The impact of the processing conditions of the edible Undaria pinnatifida brown seaweed on the dehydration and bioactive features was determined using the green microwave hydrodiffusion and gravity technology. For the alga solid phase, the corresponding dehydration and rehydration kinetics, their mathematical modelling, volumetric shrinkage, energy consumption, microscopical and colorimetric properties, phenolic content and antioxidant capacity were analyzed. Results indicated that an increase in microwave power involved a notable drop in the seaweed dehydration time. The shortest dehydration treatment to reach an adequate final moisture content (< 10% dry basis) was achieved using the following sequential procedure: 300 W (5 min), 100 W (10 min), 50 W (35 min) and 25 W (90 min). This recommended process also led to short rehydration time (10 min), uniform tissue surfaces, low total color differences and adequate bioactives contents. Experimental dehydration and rehydration kinetics were adequately fitted using Page and Peleg models, respectively. Estimated energy requirements were dramatically lower when compared to conventional seaweed drying treatment.