Hyperpolarized 13C MRI leverages an over 10 000 times increase in the 13C polarization of biomolecules, enabling new molecular imaging applications. However, metabolic applications are limited to processes on the timescale of a minute by the short lifetime of the hyperpolarized signal. Here, we hyperpolarize nanodiamonds and microdiamonds to achieve large, long-lived 13C polarizations. We then image these particles in phantoms, demonstrating the potential of hyperpolarized nanodiamonds for imaging over long timescales. As nanodiamond has been established as a biocompatible platform for drug delivery, our results will motivate further research into hyperpolarized MRI for tracking nanoparticles in vivo.