A new microwave-assisted hydrothermal carbonization (MAHC) method is reported in this work. The process uses microwave heating at 200 °C in acidic aqueous media to carbonize pine sawdust (Pinus sp.) and α-cellulose (Solucell®) at three different reaction times. Elemental analysis showed that the lignocellulosic samples subjected to MAHC yielded carbon-enriched material 50% higher than raw materials. Increase in aromaticity was confirmed by a van Krevelen diagram. SEM micrographs detected no morphological changes in pine sawdust. In contrast, SEM micrographs of carbonized α-cellulose revealed spherical-shaped particles with diameters ranging from 1 to 2 μm. These results showed that microwave-assisted hydrothermal carbonization is an innovative approach to obtain carbonized lignocellulosic materials.