Ethnic nationalism is a common phenomenon in Niger Delta, Nigeria. Ethnic nationalism in Niger Delta has been agitating for so long for self-determination and to control their God giving resources, in their quest to achieve their aims, their actions sometimes has generated a lot of tension to Niger Delta region. The study examined ethnic nationalism and internal security in Niger Delta region, Nigeria. The study examined the research questions. Has ethnic nationalism affect the internal security of Niger Delta region 1999-2014? Secondly has Nigerian government institutions curtailed violent Niger Delta ethnic nationalism 1999-2014? Group theory was adopted as our theoretical framework. The study relied on secondary source of data and as such generated qualitative data. The study revealed that ethnic nationalism has affected the internal security of Niger Delta region, Nigeria 1999-2014. Secondly, Nigeria government institutions has not adequately curtailed violent Niger Delta ethnic nationalism in the region 1999-2014.Based on the findings, this study is of the view that government should promote peace as the foundation for development, improve and diversify the economy, promote environmental sustainability to preserve the means of people sustainable livelihood, build sustainable partnerships for the advancement of human development and Nigeria government institutions should cut off from the Nigeria syndrome.