This paper is devoted to the study of sets of finite perimeter in RCD.K; N/ metric mea-sure spaces. Its aim is to complete the picture of the generalization of De Giorgi's theorem within this framework. Starting from the results of Ambrosio et al. (2019) we obtain uniqueness of tan-gents and rectifiability for the reduced boundary of sets of finite perimeter. As an intermediate tool, of independent interest, we develop a Gauss-Green integration-by-parts formula tailored to this setting. These results are new and non-trivial even in the setting of Ricci limits.
