  This note is devoted to the study of sets of finite perimeter over RCD$(K,N)$
metric measure spaces. Its aim is to complete the picture about the
generalization of De Giorgi's theorem within this framework. Starting from the
results of [2] we obtain uniqueness of tangents 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.
