A foldable brake pedal apparatus for autonomous vehicles provides a manual driving mode and an autonomous driving mode. In the manual driving mode, a driver directly drives a vehicle, a pedal pad protrudes from a pedal housing and pops up to be exposed to the driver side such that the driver can manipulate the pedal pad. And in the autonomous driving mode, the driver does not directly drive the vehicle, the pedal pad is inserted into the pedal housing and is in a hidden state in which exposure of the pedal pad toward the drive side is inhibited such that the driver cannot manipulate the pedal pad.