Cultural heritage sites are exposed to a variety of risks, for instance, by robbery, vandalism, harming, terrorism, and cyber attacks, which might damage people and cultural heritage place. For this motive, it is necessary to plan appropriate countermeasures to prevent the above risks and to protect them using intrusion detection, access control, video surveillance, communication systems, cybersecurity devices and solutions, security personnel, and procedures properly mixed to attain an integrated system or solution. In this paper, a new security risk assessment method for cultural heritage sites (SRACHS) is presented, showing as a case study, without any loss of its wide pertinence, its application to a museum. Further, a proper genetic algorithms (GAs)-based methodology to optimize risk reduction countermeasures is presented. The proposed security risk assessment methodology allows for obtaining the correct amount of security defences (intrusion detection system, access control, video surveillance, communication devices, security personnel, etc.) that a desired cultural heritage place necessitates and the associated characteristics which depend on the probable targets that can be attacked. It also avoids of overestimating the risks as in the situation of planning unnecessary protective countermeasures that sometimes cannot be needed, thus reducing the connected extra expenses, as properly demonstrated by the GAs-based methodology to optimize risk reduction countermeasures proposed in this paper.