This study examined the effect of pyrolysis temperature on the physicochemical properties of municipal sewage-sludge biochar and on the speciation of its heavy-metal constituents. Dewatered sludge was carbonised at 300 °C, 400 °C, 500 °C, 600 °C and 700 °C, and the resulting products were characterised to identify an optimal operating temperature. Higher temperatures enlarged the pore network of the biochar and caused pronounced organic-carbon loss, imparting a distinctly alkaline character. Pyrolysis removed more than 80% of the nitrogen, whereas total phosphorus and potassium concentrations increased; their plant-available fractions declined overall, yet remained comparatively high at 500 °C. Most heavy metals were retained in the residual fraction and became further immobilised during pyrolysis, thereby lowering bioavailability, although Pb and Ni showed increased mobility at 600–700 °C. Cadmium also persisted mainly in the residual form, but its elevated concentration still limits agricultural application of the char. Considering nutrient content, pH, and metal bioavailability, 500 °C is recommended as the optimal carbonisation temperature.