Reject brine, generated as a waste at the end of the desalination process, presents a useful source for the extraction of valuable resources. This study investigated the recovery of reactive MgO from reject brine obtained from a local desalination plant. This was enabled via the reaction of Mg2 + present within reject brine with an alkali source (NaOH), which led to the precipitation of Mg(OH)2, along with a small amount of CaCO3. The determination of the optimum NaOH/Mg2 + ratio led to the production of the highest amount of yield. The synthesized Mg(OH)2 was further calcined under a range of temperatures (500–700 °C) and durations (2 − 12h) to produce reactive MgO. A detailed characterization of MgO obtained under these conditions was presented in terms of its reactivity, specific surface area (SSA), composition and microstructure. While an increase in the calcination temperature and duration decreased the reactivity and SSA of MgO, samples calcined at 500 °C for 2 h revealed the highest reactivity, which was reflected by their SSA of 51.4 m2/g.