Thermomechanical treatments for manipulating grain boundary microstructure in 10 wt%Cr ferritic-martensitic steel SUH3 have been studied. Material with a high fraction of coincidence site lattice (CSL) boundaries was successfully produced and subjected to steam oxidation tests to demonstrate the utility of grain boundary engineering. Introducing a high fraction of twin boundaries in austenite resulted in a significant increase in the number of CSL boundaries along the prior austenite grain boundaries in martensite. In addition, grain boundary engineering introduced a high density of subblock structures in martensite, which resulted in a homogeneous distribution of fine precipitates in tempered martensite. Steam oxidation tests demonstrated that grain boundary engineering for SUH3 steel can achieve enhanced oxidation resistance.