The hippocampus (HPC) is required for the initial process of permanent memory formation. After memory acquisition, HPC-dependency of memory recall gradually decreases with time, whereas cortical-dependency of memory recall increases. This phenomenon is often referred to as systems consolidation of memory. In the HPC, adult neurogenesis has been described in many mammalian species and is involved in the process of learning and memory. In this review, we first examine the neural circuit mechanisms that underlie the systems consolidation of episodic memories, focusing on adult neurogenesis within the HPC. Recently, several studies have examined the characterization of memory engram cells, their biochemical and physiological changes, and the circuits for systems consolidation of memory. Therefore, we discuss the possible role of adult neurogenesis on functional states of memory engram cells, and speculate the implications of this transition of memory type for psychiatric disorders such as post-traumatic stress disorder.