In this paper, we study a backward problem for an inhomogeneous fractional diffusion equation in a bounded domain. By applying the properties of Mittag-Leffler functions and the method of eigenvalue expansion, we establish some results about the existence, uniqueness, and regularity of the mild solutions as well as the classical solutions of the proposed problem in a weighted Hölder continuous function space. KEYWORDS backward problem, existence, fractional diffusion equation, regularity MSC CLASSIFICATION 26A33; 35R11 d dt ∫ t 0 (t − s) −1 u(s, x)ds, t > 0, Math Meth Appl Sci. 2019;42:6775-6790.wileyonlinelibrary.com/journal/mma