We present a 300 GHz fundamental PLL, based on a 300 GHz VCO, 2:1 dynamic frequency divider, fifth-order sub-harmonic phase detector, and active loop filter, fabricated in an InP HBT technology. The PLL achieves locking from 300.76 to 301.12 GHz, with -23 dBm of output power and -78 dBc/Hz of phase noise at a 100 KHz offset, while consuming 301.6 mW. The PLL occupies 0.84 mm 2 including pads. This work represents the highest frequency PLL reported thus far, 2× to 3× faster than previously reported PLLs.