The recycling of Municipal Solid Waste Incineration Bottom ash (BA) with a size below 4 mm is often difficult, due to its high contmination and porosity. To increase its reuse, the manufacture of lightweight aggregates represents a suitable option, as it takes advantage of the BA porous structure and uses cement for the immobilization of harmful contaminants. For highly contaminated ashes, hydrothermal treatment (HT) is known for further improving the immobilization of some heavy metals in the cementitious matrix. However, not many studies have investigated it as a curing method for pellets. This study addresses the application of highly contaminated BA fines in the production of lightweight aggregates. Firstly, pellets containing up to 80%wt. BA are manufactured and classified as lightweight aggregates, according to EN 13055. In the absence of HT, the formation of Friedel's salt helps the Cl− retention, lowering the leaching by 65% compared to theoretical values. After HT, the formation of tobermorite on the pellet surface is beneficial for the flexural strength (+14%). However, the decomposition of phases such as Friedel's salt increases the leaching of Cl−, Cu, Mo, and Zn compared to standard cured samples.