We report on the motional behavior of the tetrahedral interstitial muonium (Mu T) the chemical analogue, and light isotope of neutral atomic hydrogen, in a 90% enriched 13C diamond. Muonium forms after implantation of positive muons into diamond. In the 13C diamond, the muon spin relaxes due to the motion of Mu T against the background of randomly oriented spins of the host nuclei. Spin-lattice relaxation of the initially spin-polarized muons, in their capacity as nuclei of the Mu T atoms, was studied as a function of magnetic field and temperature in the ranges 20 mT–0.4 T and 10–400 K, respectively. The extracted hop rate is large, τ c −1∼10 11 Hz, indicating fast diffusion of Mu T, as expected for such a light interstitial atom. Its approximately constant value over the entire measured temperature range suggests weakly scattered coherent tunneling rather than incoherent or phonon-assisted tunneling, consistent with the high Debye temperature of diamond and the shallow interstitial potential for neutral hydrogen or muonium. There are indications of increasing localization of the wave-packet, i.e., of an incipient change to incoherent tunneling, towards the higher temperatures. Thermal conversion of the mobile Mu T state to the stable bond-centered muonium (Mu BC) state prevents the diffusion of Mu T being explored beyond 400 K.