Frederick James Hargreaves (10 February 1891 – 4 September 1970) was a British astronomer and optician.

He was considered the foremost optician in Britain, and was noted for his skill in mirror making and other optics for astronomical telescopes. He also made a series of observations and drawings of the turbulence in the belts and zones of Jupiter's atmosphere.

He served as president of the British Astronomical Association (1942-1944).

He partnered to create the firm Cox Hargreaves and Thompson, with first premises under the embankment of the mainline station near Preston Park, Brighton, Sussex. Later, moving to the much better environment for really accurate grinding and surface continuity at the large tunnel layout (Deep Level Air Raid Shelter No. 4), in the grounds of Cane Hill Asylum, Coulsdon, Surrey, from 1952-1970. Cut deep into a chalk hill, these brick-lined tunnels had an all-year round constant temperature and absolutely no vibration. Mirrors tested at point of manufacture and later using modern testing equipment have shown Hargreaves' hand-cut mirrors to be accurate to an average of 3 wavelengths of sodium light.

Hargreaves trained telescope-maker Norman Fisher (1936-1994) in mirror making at the Cane Hill tunnels. The Norman Fisher Observatory is sited at Kenley, Surrey.