A diplomatic frequency was a frequency in the subspace bandwidth, reserved for diplomatic communications.

The Vulcans used a diplomatic frequency that was located in the lower band in the 22nd century. While chased by hostile Mazarite ships, Ambassador V'Lar used this frequency on board of the Enterprise NX-01 to send the combat cruiser Sh'Raan a message in 2152. The frequency, unbeknown to the Mazarites, got through their jamming signal. (ENT: "Fallen Hero")