An alkali-mediated hydrothermal approach was optimized for partial extraction of high-purity hemicellulose (xylan) from a timber waste product (unbleached eucalyptus sawdust). The extraction process was: a) extractives removal using sequential solvent extraction; b) alkali-mediated hydrothermal extraction of hemicellulose; and c) purification of the isolated hemicellulose fraction. Biomass was extracted in an autoclave at a fixed temperature of 121 °C while varying the extraction time (1 h to 2.5 h) and alkali concentration (10%, 15%, and 20% w/v). Using this simple approach, high purity polymeric hemicellulose with molecular mass (21 to 30 kDa) was recovered in yields of 35 to 37%. The isolated hemicellulose can be chemically transformed into high-value commercial products such as prebiotics (xylooligosaccharides), surfactants, hydrogels, and food packaging materials. The purified residue can be utilized in existing kraft pulping processes. The integration of hemicellulose extraction into existing pulping process represents additional valorization of timber resources.