Legal security is the main pillar to achieve and build sustainable peace in post-conflict societies, especially after these societies were exposed to economic, social and environmental crises in addition to armed conflicts, as the idea of legal security emerged in this field as a means of moving from a state of chaos to a state of order and security and then sustainable peace. This research aims to shed light on the relationship between law security and sustainable peace and to show that legal security is a just and stable system that aims to prevent the occurrence of conflicts and future crises in societies, protect states, try to reach sustainable peace, achieve justice and protect human rights, as the research focuses on reviewing the difficulties and challenges facing legal security in its application in societies that have been exposed to the collapse of the security system due to conflicts and Wars, many issues have been addressed in countries that are still suffering from the collapse of legal security, including Syria and Iraq, and on the other hand, the research dealt with some successful examples that enable legal security to achieve sustainable peace, including Bosnia and Herzegovina and also Rwanda. Achieving sustainable peace through the application of legal security requires the intervention of the rule of law and the role of the international community in supporting legality to achieve transitional justice and stability of societies.