Generalizing the work of Lenard and Bernstein, we introduce a new, fully relativistic model to describe collisional plasmas. Like the Fokker–Planck operator, this equation represents velocity diffusion and conserves particle number. However, unlike the Fokker–Planck operator, it is linear in the distribution function, and so more amenable to a fluid treatment. By taking moments, we derive a new fluid model together with a hierarchy of closure schemes, and demonstrate the damping effects of collisions on Langmuir waves in cold and warm plasmas.