Abstract Background To prevent sexual boundary violations (SBV) in mental health care institutions overall governments require these institutions to report SBV incidents to a central registry and to develop institutional guidelines how to react. In Europe SBV policies are only recently developed or implemented, as is also the case in Flanders (Belgium). The implementation of a new institutional policy is always a challenge and can encounter resistance, especially when it concerns SBV, because they remain delicate and complex. Method This study evaluated the extent to which mandatory policies on SBV have been implemented in mental health care institutions in Flanders, and possible factors for (non-)implementation of these policies. An online survey was sent to the executives of all mental health care institutions in Flanders (N = 162). Results In total 56 executives of mental health care institutions filled out the survey (response rate 35%). Results showed that the implementation of an SBV policy in mental health care institutions is unfortunately inadequate and not all SBV incidents were reported to the central registry. Type of institution and opinions on the SBV policy were related to the (non-)implementation of the requirements. Conclusions It is recommended that governments regularly communicate with mental health care institutions to better understand the concerns and difficulties concerning implementation of the required SBV policy and to support/stimulate an organisational culture of more openness and safety on this topic.