The signal box stands immediately south of Harlech station, adjacent to the down loop. It is a two-storey brick building under a pitched slated roof.
The box is a Great Western Railway type 8a, dating from the early 1920s, constructed in red brick with black brick plinth cappings and window sills and clad with horizontal boarding above window level to the roof on the trackside elevation and gables. The box measures 7.64m north to south by 3.48m. The locking room is unusually well-endowed with windows: there are three 6-light cast iron frames in the east wall, three more on the west side and one on the south wall.
A 30-lever GWR VT-3 frame is installed, although only 23 levers are fitted and only five of those were in use at the time of the visit just prior to planned closure two days later; Tyer's key token block instruments are in use. (NB: The box has since been demolished).
For further information on the types and designs of signal boxes see: The Signalling Study Group, The Signal Box - A Pictorial History and Guide to Designs, OPC, 1986.

B.A.Malaws, RCAHMW, 21 October 1988.