Sir William Nosworthy KCMG (18 October 1867 – 26 September 1946) was a New Zealand politician. He briefly served as Minister of Finance and then Minister of External Affairs in the Reform Government.
Nosworthy stood in the Ashburton electorate in the 1908 election. The Second Ballot Act was in force and on election night, he was in third place behind David Jones. Nosworthy encouraged his supporters to vote for Jones in the second ballot. After a recount, the situation was reversed (Nosworthy was 30 votes ahead) and Jones advertised for his supporters to vote for Nosworthy, and indeed, Nosworthy was successful in the second ballot.
Nosworthy held this electorate until his retirement at the 1928 election, serving through six terms of Parliament.
He was first appointed to Cabinet in 1919, being appointed Minister of Agriculture (1919–1925) and then Minister of Immigration (1920–1925) by William Massey. When Massey died in 1925, Nosworthy gained the more important role of Minister of Finance in the interim government of Francis Bell.