Organic semiconductors offer a long-spin coherence time and diffusion length due to the weak spin-orbit and hyperfine interactions in these materials. However, in commonly used lateral field-effect transistor structures, it is challenging to define device dimensions comparable to the spin diffusion length. On the other hand, vertical structures, offering smaller device dimensions, are facing issues due to the low carrier mobilities in the vertical dimension. Here, we investigate spin relaxation in rubrene thin films with a triclinic phase, which are doped with C60F48 by coevaporation. The doping provides an efficient way to generate charge carriers, and their high out-of-plane mobility should enhance long-spin diffusion. Using electron-spin resonance, we show that the spin relaxation is governed by the interaction with the dopant counterions and estimate the spin diffusion length to be ∼200 nm. This is comparable to the film thickness, which should make such doped rubrene films an attractive system for spintronic device applications.