Wood is a crucial construction material. However, it usually requires protection against biological agents through preservative impregnation. Certain wood species have low permeability, which poses difficulties for impregnation. Eucalyptus and maritime pine are relevant wood species in Portugal due to their prevalence and economic importance. Among the different modification processes of wood, the microwave (MW) treatment is being researched as a method to improve wood’s impregnability. Thus, this study aimed to improve the impregnability of both Portuguese wood species with MW treatment, using small clear wood specimens. Results showed significant improvement in preservative retention of MW-treated samples, allowing for application in Use Classes 1, 2, 3, and 4, with special emphasis on the remarkable increase in preservative retention of MW-treated eucalyptus compared to control ones. Finally, MW treatment has proven to be a valuable technique for enhancing impregnability in maritime pine and overcoming low permeability in eucalyptus wood. The improved impregnability resulting from MW treatment offers great potential for expanding the use of these wood species in construction applications.