Hate speech as a social phenomenon inundates the social, economic and political life of many nations of the world. Evidence from the literature points to its negative impacts on intergroup relations and national security. In recent times, incidents of hate speeches are on the increase in Nigeria. This paper examines the evolution and development of hate speech, the causal factors and the implications on intergroup relations and national security in Nigeria. The study is hinged on the social identity theory of intergroup relations. The author researched various literature on the subject matter and employed participant observation to arrive at the set conclusion. The paper posits that hate speech in Nigeria is traceable to illiteracy, poor social orientation, ethnicity, religious cleavage and activities of the political class. Hate speech encourages hatred, promotes disunity, violence, a threat to national integration and poses serious security challenges. The paper suggests among others social re-orientation of the citizens, the need for the political class and the traditional institutions to be alive to their social responsibility, promotion of ethnic and religious tolerance and enactment of strict laws prohibiting hate speech in the country.