We report on preparation, NMR and MRI study of suspensions of detonation nanodiamonds (DNDs) with manganese ions directly grafted to their surfaces, which are considered as novel contrast agents for MRI. We measured proton relaxation times, relaxivities and MRI images in aqueous and saline suspensions of manganese-grafted nanodiamonds, and those covered by polyvinylpyrrolidone, with different manganese concentrations. A comparison of the above parameters in Mn-grafted DNDs with those in the recently studied Gd-grafted DNDs and Dotarem, a contrast agent usually used in clinical practice, leads to a conclusion that Mn-grafted DNDs may serve as contrast agents for MRI.