jats:titleAbstract</jats:title>jats:pDomain wall motion is detected for the first time during the transition to a ferroelastic and spin state ordered phase of a spin crossover complex. Single‐crystal X‐ray diffraction and resonant ultrasound spectroscopy (RUS) revealed two distinct symmetry‐breaking phase transitions in the mononuclear Mnjats:sup3+</jats:sup> compound [Mn(3,5‐diBr‐saljats:sub2</jats:sub>(323))]BPhjats:sub4</jats:sub>, 1. The first at 250 K, involves the space group change jats:italicCc</jats:italic>→jats:italicPc</jats:italic> and is thermodynamically continuous, while the second, jats:italicPc</jats:italic>→jats:italicP</jats:italic>1 at 85 K, is discontinuous and related to spin crossover and spin state ordering. Stress‐induced domain wall mobility was interpreted on the basis of a steep increase in acoustic loss immediately below the the jats:italicPc</jats:italic>‐jats:italicP</jats:italic>1 transition</jats:p>