The paper examined the changing nature of the National Security threats in the 21st century using Nigeria’s Fourth Republic as a case.  Insecurity troubles contemporary societies all around the globe. Nigeria like every other nation-state is not exempted from national security challenges. Since the inception of the Fourth Republic in 1999, the monopoly of the legitimate use of violence of the Nigerian State has been contested by non-state actors in various parts of the country. They include the Niger Delta militia, Boko Haram, Eastern Security Network, Bandits, and Answal, among others. The scenario has created ungovernable space in various parts of the country leading to attacks and kidnapping for ransom. Besides, other human security challenges include climate change leading to flooding and erosion, desertification leading to pastoral conflicts, electoral violence, COVID-19 pandemic, among others.  The paper adopted securitization as its theoretical construct. The paper relied on a qualitative method of data gathering technique. The paper unraveled that some of the challenges are triggered by political class promoting religion and ethnic sentiment which become a national security problem. The paper recommended amongst others to call for national dialogue and improve intelligence gathering and sharing among security agencies.