This communication presents the first Gd(iii)-based T1 MRI contrast agent, o-MeHgGad, for noninvasive visualization of CH3Hg+. o-MeHgGad showed a relaxivity enhancement of 62% in the presence of 1 equiv. of CH3Hg+. Moreover, a noticeable contrast enhancement was recorded in the liver, kidney, and intestine of mice exposed to CH3Hg+. Thus, the newly designed contrast agent has the potential to be used for in vivo bio-imaging of CH3Hg+ and could be useful for biomedical applications.