Recovering materials with high value and increasing market demand from sewage and/or sludge is becoming more attractive than recovering traditional resources such as nutrients and biogas. Sulfated polysaccharides (SPs) are valuable and scarce raw materials that can only be produced from marine algae and a few types of animal tissues. This study evaluated if SPs are present in activated sludge obtained from saline sewage with a high level of sulfates present. The presence of SPs-containing extracellular polymeric substances (EPS) was confirmed and quantified for both sludge from lab-scale reactors and full-scale plants for the first time. SPs in the sludge of a lab-scale reactor operated under alternating aerobic/anoxic conditions with 500 mg/L sulfate in the influent (which is typical of Hong Kong saline sewage) reached 342.8 ± 0.3% mg/gVSS, and sludge taken from a full-scale saline wastewater treatment plant (WWTP) contained 418.1 ± 0.4% mg/gVSS of SPs. Purity of the extracted SPs was comparable to that of commercial industrial-grade products. Key bioactivities of SPs (i.e. fucoidan, carrageenan and heparin), namely anti-angiogenesis, anticoagulant and antioxidant, were confirmed after extraction and purification. Interestingly, operating conditions had a strong influence on the contents and types of SPs synthesized in sludge as well as its bioactivities. Although the detailed synthetic pathways of SPs in activated sludge remain unclear, the current study has made a first attempt to recover a high–value scarce resource from biological wastewater treatment.