William Frederic Sayer KC (3 December 1857 – 26 February 1943) was an Australian lawyer and politician who was a member of the Legislative Assembly of Western Australia from 1901 to 1902.

Sayer was born in London, and attended the University College School before going on to study law.

[1] In March 1901, despite not being a member of parliament, Sayer was appointed attorney-general in the ministry of George Throssell (succeeding Richard Pennefather).

[2] At the 1901 state election, a month after being elevated to the ministry, Sayer won the newly created seat of Claremont.

In December 1902, Sayer was appointed crown solicitor (equivalent to Solicitor-General) in succession to Robert Bruce Burnside, a position which he held until his retirement in 1930.