Transcription factor CREB is believed to play essential roles in the formation of long-term memory (LTM), but not in learning and short-term memory (STM). Surprisingly, we previously showed that transgenic mice expressing a dominant active mutant of CREB (DIEDML) in the forebrain (DIEDML mice) demonstrated enhanced STM and LTM in hippocampal-dependent, rapid, one-trial learning tasks. Here we show that constitutive activation of CREB enhances hippocampal-dependent learning of temporal association in trace fear conditioning and delayed matching-to-place tasks. We then show that in DIEDML mice the apical tuft dendrites of hippocampal CA1 pyramidal neurons, required for temporal association learning, display increased spine density, especially of thin spines and of Homer1-negative spines. In contrast, the basal and apical oblique dendrites of CA1 neurons, required for rapid one-trial learning, show increased density of thin, stubby, and mushroom spines and of Homer1-positive spines. Furthermore, DIEDML mice showed increased dendritic complexity in the proximal portion of apical CA1 dendrites to the soma. In contrast, forebrain overexpression of CaMKIV, leading to enhanced LTM but not STM, show normal learning and CA1 neuron morphology. These findings suggest that dendritic region-specific morphological changes in CA1 neurons by constitutive activation of CREB may contribute to improved learning and STM.