Diffusion of Li⁺ in (111) oriented thin films of the spinels Li₄Ti₅O₁₂ and LiTi₂O₄ has been studied with ⁸Li β-detected NMR in the temperature range between 5 and 310 K. In Li₄Ti₅O₁₂, the spin-lattice relaxation rate (1/T₁) versus temperature shows a clear maximum around 100 K (=T_max) which we attribute to magnetic freezing of dilute Ti³⁺ local magnetic moments, consistent with the results of magnetization and muon spin relaxation (μ⁺SR) measurements. The decrease in 1/T₁ with temperature above T_max indicates that Li⁺ starts to diffuse with a thermal activation energy (E_a) of 0.11(1) eV. In LiTi₂O₄, on the contrary, as temperature increases from 200 K, 1/T₁ increases monotonically up to 310 K. This suggests that Li also starts to diffuse above 200 K with E_a = 0.16(2)eV in LiTi₂O₄. Comparison with conventional Li-NMR on Li₄Ti₅O₁₂ implies that both β-NMR and μ⁺SR sense short-range Li motion, i.e., a jump diffusion of Li⁺ to the nearest neighboring sites.