AbstractNanodiamonds (ND) hold great potential for diverse applications due to their biocompatibility, non‐toxicity, and versatile functionalization. Direct visualization of ND by means of non‐invasive imaging techniques will open new venues for labeling and tracking, offering unprecedented and unambiguous detection of labeled cells or nanodiamond‐based drug carrier systems. The structural defects in diamonds, such as vacancies, can have paramagnetic properties and potentially act as contrast agents in magnetic resonance imaging (MRI). The smallest nanoscale diamond particles, detonation ND, are reported to effectively reduce longitudinal relaxation time T1 and provide signal enhancement in MRI. Using in vivo, chicken embryos, direct visualization of ND is demonstrated as a bright signal with high contrast to noise ratio. At 24 h following intravascular application marked signal enhancement is noticed in the liver and the kidneys, suggesting uptake by the phagocytic cells of the reticuloendothelial system (RES), and in vivo labeling of these cells. This is confirmed by visualization of nanodiamond‐labeled macrophages as positive (bright) signal, in vitro. Macrophage cell labeling is not associated with significant increase in pro‐inflammatory cytokines or marked cytotoxicity. These results indicate nanodiamond as a novel gadolinium‐free contrast‐enhancing agent with potential for cell labeling and tracking and over periods of time.