Declarative memory including semantic and episodic memories refers to those memories that can be explicitly stated or consciously recalled. The declarative memory has been considered to be crucial for communications in our daily life and for the development of human culture. The medial temporal lobe regions are known to be critically involved in the formation and maintenance of declarative memory. Especially, prior studies have suggested that declarative learning enhances neural activity in the hippocampus and that such neural activity is associated with memory consolidation processes. However, it is still unclear whether neural representations of stimuli change during learning, and whether the change in neural representations reflects retention of the declarative memory. Herein, we combined an event-related functional magnetic resonance imaging and association memory paradigms to investigate the relationship between the change in neural representations during declarative learning and the subsequent memory retention in the hippocampus. Using trial-by-trial similarity analysis during learning, we found that between-stimulus pattern similarity for associated stimuli significantly increased during learning in the left hippocampus. Moreover, the change in representational similarity between associated stimuli was significantly correlated with the subsequent memory performance at retrieval one day or four weeks after learning. The same tendency was also found in the entorhinal cortex of the medial temporal lobe regions, especially for the memory performance at retrieval four weeks after learning. In the visual cortex, the change in representational similarity was not correlated with the subsequent memory performance while pattern stability for the same stimulus was correlated with the subsequent memory performance. These results suggest that the change in neural representations in the medial temporal lobe regions, especially the hippocampus, during learning reflects the maintenance of the declarative memory.