Standard diffusion is described in terms of a parabolic partial differential equation, while relativity is governed by hyperbolic differential equations, due to the constancy of the speed of light. Thus there is no obvious generalization.
Entry points for various ways to generalize the notion are the following papers, in reverse chronological order. (The old papers are still worth reading.)
O'Hara, P., & Rondoni, L. (2015). Brownian motion in Minkowski space. Entropy 17, 3581-3594.
Terras, A. (2013). The Poincaré Upper Half-Plane. In Harmonic Analysis on Symmetric Spaces—Euclidean Space, the Sphere, and the Poincaré Upper Half-Plane (pp. 149-376). Springer New York.
Baeumer, B., Meerschaert, M. M., & Naber, M. (2010). Stochastic models for relativistic diffusion. Physical Review E 82, 011132.
Dunkel, J., & Hänggi, P. (2009). Relativistic Brownian motion.Physics Reports 471, 1-73. https://arxiv.org/abs/0812.1996
Haba, Z. (2009). Relativistic diffusion. Physical Review E 79, 021128.
Kostädt, P., & Liu, M. (2000). Causality and stability of the relativistic diffusion equation. Physical Review D 62, 023003.
Posilicano, A. (1997). Poincaré-invariant Markov processes and Gaussian random fields on relativistic phase space.Letters in Mathematical Physics 42, 85-93.
Dudley, R. M. (1966). Lorentz-invariant Markov processes in relativistic phase space. Arkiv för Matematik, 6, 241-268. (See also here.)
Hunt, G. A. (1956), Semi-groups of measures on Lie groups, Transactions o/the American Mathematical Society 81, 264-293.