Based on the perspectives of popular nationalism and state nationalism, this article investigates the current status of nationalism as well as its effects on the regional security order in Northeast Asia. The rise of nationalism in Northeast Asia is multifaceted and presents a dynamic development trend, which has a profound impact on the regional security order in Northeast Asia. Although the popular nationalism of Northeast Asian nations has various effects on the diplomatic decision‐making of the respective governments, all of these versions of nationalism affect normal communications among nations in the region. Thus, each government should be careful when utilizing nationalism to realize its political purposes. Indeed, Japan and China should learn from the lessons of history in World War II and avoid falling into the abyss of state nationalism. Each nation in Northeast Asia should pay more attention to the rise of nationalism in the region, constrain its narrow‐mindedness and exclusiveness, and aspire to rational patriotism. By doing so, a peaceful, secure, and prosperous regional security order in Northeast Asia could be constructed.