The time-dependent relaxation of retained polarization in unipolar methylammonium bromobismuthate, (CH₃NH₃)₅Bi₂Br₁₁, single crystal was examined by optical observation of the domain structure. The polarization relaxation is attributed to the presence of a built-in internal bias field caused by structured disorder in the crystal. The internal field accounts for the broad spectrum of potential barriers for the domain walls. The slow polarization decay, in the long-time regime (t > 1 s), follows a stretched exponential and/or power-law function. On the assumption that relaxation centers (domains) are noninteracting, a distribution function of relaxation times is reconstructed.

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