Abstract: Memory and spatial navigation are complex functions that involve multiple brain regions. We study one such brain region – the retrosplenial cortex. Though there is compelling evidence for the role of the retrosplenial cortex in spatial memory formation and retrieval, little is known about how this brain region executes these functions. To understand the neural code employed by the retrosplenial cortex, we performed large-scale electrophysiological recordings during sleep and wake states. Here, we show that there exists a novel, fast, 140 Hz brain rhythm in the retrosplenial cortex during REM sleep and active behaviors. These rhythms demarcate high activity frames during REM sleep. Using whole cell physiology and computational modeling, we show that these retrosplenial rhythms are mechanistically distinct from previously recognized fast rhythms such as ripples and gamma. Finally, we show that retrosplenial 140 Hz rhythms are precisely modulated by running speed, highlighting their central role in shaping the critical navigation and spatial memory functions of the retrosplenial cortex.