The focus of this study was to characterize the surface changes of several species of thermally modified wood after plasma treatment with diffuse coplanar surface barrier discharge (DCSBD). Surfaces were characterized before and after the DCSBD plasma exposure with different treatment time by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), confocal microscopy and contact angle measurements to determine the surface free energy. Surface free energy of 63–64 mJ/m2 was measured for all thermally modified wood species already after 5 and 10 s of plasma treatment. The increased surface free energy could be attributed to the increase of the polar component of surface free energy. XPS measurements revealed that the oxygen to carbon atoms (O/C) ratio increased from 0.2 to 0.3 before plasma treatment up to 0.7–0.8 after plasma treatment. A linear correlation could be suggested between the surface free energy and O/C ratio of thermally modified wood before and after plasma treatment. SEM imaging revealed that on the micrometer scale the surface morphology did not change after the plasma treatment. However, nanoscale structuring was observed at higher magnification after longer plasma treatment of 60 s.