The aim of this study was to determine the possibility of using fly ash for the solidification/stabilization (S/S) of Ni and Zn contaminated sediment from the Krivaja river basin (Serbia), which represents an extraordinary risk to the environment and belongs to the last quality class in the Dutch classification. This class of sediment needs dredging and remediation treatments since the dredged material is too contaminated and cannot be used directly. S/S treatment has proven to be effective for the remediation of wastes contaminated with metals and can lead to the beneficial and economically feasible use of sediment. Fly ash was used as a stabilising agent because it occurs as a secondary industrial product, so in this way two types of waste are immobilized. In addition, pozzolanic-based S/S is an effective and economic remediation technology to immobilize metals in contaminated soils and sediments. In order to determine the long-term behaviour of the S/S mixture, the semidynamic ANS 16.1 leaching test was performed. The existing leaching method was modified and acetic acid and humic acid were also used as leachants in order to simulate possible “worst case” leaching conditions for S/S waste being disposed of in a landfill environment (acid rain, floods, etc.). S/S effectiveness was evaluated by measuring the cumulative fractions of metals leached, effective diffusion coefficients (D_e) and leachability indices (LX). Overall, the test results indicated that S/S treatment was effective in immobilizing Ni and Zn and the treated sediment can be considered acceptable for “controlled utilization” based on LX values. The controlling leaching mechanism was found to be diffusion in all treated samples.