To gather further data about metal mobility in accumulated sediments in stormwater treatment facilities, metal mobility in sediments from sedimentation tanks, gully pot sediment traps, and sedimentation ponds was investigated using the sequential extraction procedure. This method allows distinguishing the metal speciation between ion-exchangeable, carbonate-associated, reducible, organic matter/sulfide-associated, and residual fractions. The metal fractionation reveals that, for all treatment facilities, the majority of Cd, Cu, Pb, and Zn and a significant amount of Ni were in potentially mobile forms. Cd, Pb, and Zn are, to a large extent, associated with Fe-Mn oxides while Cu is commonly present as Cu–organic matter complexes. The metals in these potentially mobile fractions may represent a potential environmental hazard, e.g., due to release during maintenance (sediment removal) when the chemical phase distribution might change.