High-frequency brain oscillations in humans are currently categorized into beta (13–30 Hz) and gamma (>30 Hz). Here, I introduce a new class of oscillations between 25 and 35 Hz, which I propose to call “iota.” Iota oscillations have low amplitudes but can still be measured with surface electroencephalography (EEG). Within an individual, iota activity has a narrow spectral bandwidth typically less than 3 Hz, thus distinguishing it from broadband beta and gamma. Iota oscillations occur in sustained bursts during both wakefulness and rapid eye movement (REM) sleep. They are only found in a subset of individuals, more in children than in adults. Overall, iota oscillations are challenging to detect but could serve as a marker of both brain development and states of vigilance. NEW & NOTEWORTHY Electrical brain waves are some of the only neuronal signals that can be measured noninvasively in humans. Until now, only six classes of waves have been identified. Here I introduce a new class, iota, specific to wakefulness and rapid eye movement (REM) sleep. This makes iota just the second class of brain wave known to characterize REM sleep (after theta), and opens up new opportunities to investigate this elusive state.