This study examined how biochar (BC) made from willow or sewage sludge at 500 °C or 700 °C affects polycyclic aromatic hydrocarbons (PAHs) during composting of sewage sludge with wheat straw. BC addition impacted both total (Ctot) and freely dissolved (Cfree) PAHs, with effects dependent on pyrolysis temperature and, to a lesser extent, feedstock. Greater losses (both percentage and absolute) of Ctot PAHs occurred with low-temperature BC than high-temperature BC. The highest reduction was seen with sewage sludge-derived BC produced at 500 °C. For Cfree PAHs, low-temperature BCs decreased bioavailability more than high-temperature BCs, with willow-derived BCs being more effective than sewage sludge-derived BCs. Results suggest BC’s effectiveness in reducing PAHs depends mainly on feedstock for Ctot PAHs, but more on pyrolysis temperature for Cfree PAHs. BC’s capacity to reduce contaminants in compost could back regulations promoting its use in waste management and soil improvement.