HH 399 is one of the first Herbig-Haro flows recognized to be irradiated by the UV radiation of the massive O7.5 star in the Trifid Nebula. We present the proper motion of the first irradiated jet, based on two epochs of Hubble Space Telescope (HST) observations of HH 399 separated by nearly 5 yr, using Hα and [S II] line filters. High proper motion with continuous velocities between 200 ± 55 and 528 ± 24 km s-1 is detected in both lines along the 18″ extent of the jet axis. The irradiated fully ionized jet consists of numerous knots along the jet but also shows evidence for a number of isolated blob like structures running immediately outside the jet with lower transverse velocities. The transverse velocities combined with radial velocity measurements indicate that the jet axis lies away from the plane of the sky by only a few degrees. We argue that the jet is fully ionized, based on a [S II]/Hα line ratio, as well as radio continuum emission detected from the full extent of the jet at a 3.6 cm wavelength. The stellar massloss rate producing HH 399 is estimated to be ≈ 2 × 10-6 M ⊙ yr-1.