The ability to detect novel stimuli in the changing environment is critical for the survival of all animals throughout their lifetime. A central question, then, is how sensory systems in the adult brain represent novel sensory inputs and how these representations change as the stimuli become familiar. Here we address this question in the mouse olfactory bulb, where odors are represented by the activity of ensembles of mitral cells. Using chronic two-photon calcium imaging in awake mice (Komiyama et al., 2010), we show that odor familiarization by brief repeated exposure leads to a marked reduction of odor-evoked mitral cell activity without affecting sensory neuron input to the bulb. This mitral cell plasticity is odor-specific; individual mitral cells show a shift in their odor tuning to preferentially represent less-experienced odors. Experience-induced changes in odor representations recover gradually over many weeks and can be repeated with different odors. Thus, odor tuning is dynamically modified by recent experience such that unfamiliar odors strongly activate mitral cell ensembles. Ultimately, this experience-dependent plasticity provides a potential mechanism to encode novel stimuli with greater salience.