In the present report, we identified one process related impurity in brimonidine tartarate its Relative Retention Time (RRT) is 2.23. Liquid Chromatography-Mass Spectroscopy (LC-MS) showed its molecular ion peak as 248. It was isolated by preparative High Performance Liquid Chromatography (HPLC), characterized by Infra-Red (IR), Proton Nuclear Magnetic Resonance (1H-NMR) and mass spectral analysis revealed the unknown impurity as 5-Bromo-quinoxalin-6-yl-cyanamide (7). Further it was confirmed by synthesis starting from quinoxaline thiourea. Both compounds isolated by HPLC and synthesized compound confirms the unknown impurity in brimonidine tartarate as 5-Bromo-quinoxalin- 6-yl-cyanamide (7).