Reactive magnesia (MgO) with a high purity and reactivity is used in several high-end applications. This study reports the feasibility of synthesizing high reactivity MgO from reject brine with the use of NH$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$OH. The molar amount of NH$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$OH was optimized at a NH$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$OH/Mg$\mathrm{<mrow\; data-mjx-texclass="ORD">2+</mrow>}$ molar ratio of 6 to provide maximum magnesium oxide yield and purity. This led to the synthesis of Mg(OH)$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ with a purity of 93.5%, which was further calcined at 500 °C for 2 h to produce reactive MgO with a SSA of 78.8 m2/g. This study shed light on the significant potential of reject brine in the recovery of Mg$\mathrm{<mrow\; data-mjx-texclass="ORD">2+</mrow>}$ and the synthesis of reactive MgO with a wide range of potential applications.