Despite the efforts of cross-border security agencies like the Nigeria police, customs service, Immigration services, and Nigerian civil Defense etc, Nigeria borders have been described as porous allowing all sorts of cross border or transborder criminal activities such as human trafficking, smuggling, drug trafficking, arm robbery, money laundry and illicit arms trafficking resulting to proliferation of SALW. Thus, West Africa’s regional superpower, Nigeria, continues to face serious security challenges due to cross border or transborder criminal activities. We argue that; one, the spate of cross border criminal activities in West Africa undermines Nigeria’s national security; and two, frequent trans-border crimes in West African sub-region impede Nigeria’s external relations. The focus of this paper therefore is to examine the implications of cross border crimes in West Africa for Nigeria’s national security and external relations. The study is basically a historical research method relying mainly on secondary sources of data from internet sources, official documents and country websites as the method of data collection. In other words, we made use of qualitative-descriptive analysis as our method of data analysis, that is, documentary studies of official document and other materials in analyzing the secondary data. This paper titled “Cross Border Crimes in West African sub-region: Implications for Nigeria’s National Security and External Relations”. The paper is basically a historical research method relying mainly on secondary sources of data from internet sources, official documents and country websites as the method of data collection. We made use of qualitative-descriptive analysis as our method of data analysis, that is, documentary studies of official document and other materials in analyzing the secondary data. The major purpose of embarking on this research is to examine the implications of cross border crimes in West Africa for Nigeria’s