In this study, 4-mm-thick medium-density fiberboard (MDF) panels were heat-treated at 140 °C for 30 or 60 min and at 180 °C for 30 or 60 min. Then, 10-mm-thick lightweight honeycomb paperboards made from kraft paper (130 g/m2, cell diameter of honeycomb, 14 mm; compression strength, 0.21 N/mm2) were faced with the untreated and heat-treated MDF panels (thickness: 4 mm) using a two-component polyurethane adhesive. The density, thickness swelling, water absorption, and flexural properties of the paperboards faced with the untreated and heat-treated MDF panels were investigated. The lowest flexural strength (3.76 N/mm2) and flexural modulus (392 N/mm2) values were found in the specimens faced with the MDFs treated at 180 °C for 60 min, while the highest flexural strength (4.20 N/mm2) and flexural modulus (457 N/mm2) values were found in the specimens faced with the untreated MDFs. The loss in strength was primarily attributable to the degradation of hemicelluloses, which are less stable to heat than cellulose and lignin. The thickness swelling and water absorption of the honeycomb paperboards faced with the heat-treated MDF panels significantly (p < 0.01) decreased with the increase in heat-treatment temperature and duration.