Abstract. In cluster categories, mutation of torsion pairs provides a generalisation for the mutation of cluster tilting subcategories, which models the combinatorial structure of cluster algebras. In this paper we present a geometric model for mutation of torsion pairs in the cluster category C Dn of Dynkin type Dn. Using a combinatorial model introduced by Fomin and Zelevinsky in [7], subcategories in C Dn correspond to rotationally invariant collections of arcs in a regular 2n-gon, which we call diagrams of Dynkin type Dn. Torsion pairs in C Dn have been classified by Holm, Jørgensen and Rubey in [10]. They correspond to so-called Ptolemy diagrams of Dynkin type Dn, which are diagrams of Dynkin type Dn satisfying a certain combinatorial condition. We define mutation of a diagram X of Dynkin type Dn with respect to a compatible diagram D of Dynkin type Dn consisting of pairwise non-crossing arcs. Such a diagram D partitions the regular 2n-gon into cells and mutation of X with respect to D can be thought of as a rotation of each of these cells. We show that mutation of Ptolemy diagrams of Dynkin type Dn corresponds to mutation of the corresponding torsion pairs in the cluster category of Dynkin type Dn.